Replace SCSS Lint with Stylelint

SCSS Lint was based on Ruby Sass, which has been deprecated since 2018
and doesn't support some of the latest features in Dart Sass.

Since we're going to migrate to Dart Sass, we're also migrating to
Stylelint. In order to provide all the funcionality SCSS Lint had, we
also need to install a few Stylelint plugins. We also need to install
the `postcss-scss` package so Stylelint can read SCSS files.

We're still getting the linting errors we used to get in
`legislation_process.scss` because of the `max-nesting-depth` and
`selector-max-compound-selectors` rules, as well as the warnings we used
to get in `layout.scss` because the `ssb-whatsapp_app` HTML class
contains undescores in their name.

We're also getting a couple of new linting errors, which could be false
positives:

* The `scss/no-unused-private-members` rule reports errors in
  `_consul_settings.scss` because of the `$-zf-*` variables, but I'm not
  too worried about this one because these lines won't be necessary after
  updating Foundation.
* The `scss/selector-no-redundant-nesting-selector` reports an error in
  `datepicker_overrides.scss`, but removing the unnecessary nesting
  selector would make it harder to understand the code (assuming it'd
  work in the first place).

We've changed some files due to few differences between SCSS Lint rules
and their Stylelint equivalents:

* The `@stylistic/string-quotes` rule detects a case in `admin.scss`
  that StringQuotes didn't detect.
* The `function-url-quotes` rule detects a case in `mixins/icons.scss`
  that UrlQuotes didn't detect.
* The `@stylistic/declaration-bang-space-before` rule detects a case in
  `sdg/goals/show.scss` that BangFormat didn't detect.

There are also a couple of rules that don't behave exactly like they
used to:

* The equivalents of SpaceBetweenParens and SpaceAfterComma don't cover
  parenthesis or commas in mixin parameters; we haven't found rules that
  detect these cases.
* DisableLinterReason probably has an equivalent that behaves
  differently but, since we never disable linters inline, we aren't
  adding its equivalent rule.

Note we're removing the SpaceAfterVariableColon rule because its
equivalent, `scss/dollar-variable-colon-space-after`, reports cases
where we add spaces to indent several variable assignments (which we do
a lot in the `_consul_settings.scss` file). We might add this rule again
if we stop aligning consecutive assignments.

We're also removing the QualifyingElement rule because its equivalent,
`selector-no-qualifying-type: true`, behaves differently. For example,
in this code:

```
a.qualifying {
  color: inherit;
}

p {
  &.qualifying {
    color: inherit;
  }
}
```

With the QualifyingElement from SCSS Lint, the first rule is incorrect
but the second one is correct. With the selector-no-qualifying-type rule
from Stylelint, on the other hand, both rules are incorrect.

Personally, I never liked the QualifyingElement rule, and we were
working around it anyway, so we aren't applying
selector-no-qualifying-type.

For reference, here's a full list of the SCSS Lint rules we had enabled
and their Stylelint equivalents.

BangFormat
"@stylistic/declaration-bang-space-after": "never"
"@stylistic/declaration-bang-space-before": "always"

BorderZero
declaration-property-value-disallowed-list:
  border:
    - none

ColorKeyword
color-named: "never"

DebugStatement
at-rule-disallowed-list:
  - debug

DeclarationOrder
order/order:
  - dollar-variables
  - custom-properties
  - type: at-rule
    name: extend
  - type: at-rule
    name: include
    hasBlock: false
  - declarations
  - type: at-rule
    name: include
    hasBlock: true
  - rules

ElsePlacement # Apparently replaced by the combination of:
scss/at-else-closing-brace-space-after: always-intermediate
scss/at-else-empty-line-before: never
scss/at-if-closing-brace-space-after: always-intermediate
scss/at-if-closing-brace-newline-after: always-last-in-chain

EmptyLineBetweenBlocks:
  ignore_single_line_blocks: true
rule-empty-line-before:
  - "always-multi-line"
  - ignore:
    - after-comment
    - first-nested

EmptyRule:
block-no-empty: true

FinalNewline
"@stylistic/no-missing-end-of-source-newline": true

HexLength
color-hex-length: "short"

HexNotation
@stylistic/color-hex-case: "lower"

HexValidation
color-no-invalid-hex: true

IdSelector
selector-max-id: 0

ImportPath
scss/load-no-partial-leading-underscore: true
scss/at-import-partial-extension: never

Indentation
"@stylistic/indentation": 2

LeadingZero
@stylistic/number-leading-zero: "always"

NameFormat
scss/at-function-pattern: "^(-?[a-z][a-z0-9]*)(-[a-z0-9]+)*$"
scss/at-mixin-pattern: "^(-?[a-z][a-z0-9]*)(-[a-z0-9]+)*$"
scss/dollar-variable-pattern: "^(-?[a-z][a-z0-9]*)(-[a-z0-9]+)*$"
scss/percent-placeholder-pattern: "^(-?[a-z][a-z0-9]*)(-[a-z0-9]+)*$"

NestingDepth
max-nesting-depth: 4

PlaceholderInExtend
scss/at-extend-no-missing-placeholder: true

PrivateNamingConvention
scss/no-unused-private-members: true

PropertySpelling
property-no-unknown: true

PseudoElement
selector-pseudo-element-colon-notation: "double"
selector-pseudo-element-no-unknown: true

SelectorDepth
selector-max-compound-selectors: 5

SelectorFormat # Not always followed; ssb-whatsapp_app
custom-property-pattern: "^([a-z][a-z0-9]*)(-[a-z0-9]+)*$"
selector-class-pattern: "^([a-z][a-z0-9]*)(-[a-z0-9]+)*$"

Shorthand
shorthand-property-no-redundant-values: true

SingleLinePerProperty
"@stylistic/declaration-block-semicolon-newline-after": always-multi-line

SingleLinePerSelector
@stylistic/selector-list-comma-newline-after": always

SpaceAfterComma
"@stylistic/value-list-comma-space-after": always

SpaceAfterPropertyColon
"@stylistic/declaration-colon-space-after": always-single-line

SpaceAfterPropertyName
"@stylistic/declaration-colon-space-before": never

SpaceAfterVariableName
scss/dollar-variable-colon-space-before: never

SpaceAroundOperator
scss/operator-no-unspaced: true

SpaceBeforeBrace
@stylistic/block-opening-brace-space-before: always

SpaceBetweenParens
"@stylistic/function-parentheses-space-inside": never
"@stylistic/selector-attribute-brackets-space-inside": never
"@stylistic/selector-pseudo-class-parentheses-space-inside": never
"@stylistic/media-feature-parentheses-space-inside": never

StringQuotes
"@stylistic/string-quotes": double

TrailingSemicolon
"@stylistic/declaration-block-trailing-semicolon": always

TrailingWhitespace # Note it was enabled by the gem and not by us
"@stylistic/no-eol-whitespace": true

TrailingZero
"@stylistic/number-no-trailing-zeros": true

UnnecessaryMantissa
"@stylistic/number-no-trailing-zeros": true

UnnecessaryParentReference
scss/selector-no-redundant-nesting-selector: true

UrlQuotes
function-url-quotes: always

VendorPrefixes
value-no-vendor-prefix: true
selector-no-vendor-prefix: true
property-no-vendor-prefix: true
at-rule-no-vendor-prefix: true
media-feature-name-no-vendor-prefix: true

ZeroUnit:
length-zero-no-unit: true

.github/workflows/linters.yml

@@ -13,6 +13,13 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           bundler-cache: true
+      - name: Setup NPM
+        uses: actions/setup-node@v4
+        with:
+          cache: "npm"
+          node-version-file: ".node-version"
+      - name: Install node packages
+        run: npm clean-install
       - name: Run pronto
         run: |
           if [[ ${{ github.event.pull_request.head.repo.full_name }} == ${{ github.event.pull_request.base.repo.full_name }} && ${{ github.actor }} != dependabot* ]]; then