Add scss/at-mixin-no-risky-nesting-selector rule

This rule was added in stylelint-scss 6.3.0. We've always used it so
far, so we're adding it to make sure we remain using it, since it
prevents a pattern in mixins that makes the code confusing.

.stylelintrc.yml

@@ -58,6 +58,7 @@ rules:
   scss/at-if-closing-brace-newline-after: always-last-in-chain
   scss/at-if-closing-brace-space-after: always-intermediate
   scss/at-import-partial-extension: never
+  scss/at-mixin-no-risky-nesting-selector: true
   scss/at-mixin-pattern: "^(-?[a-z][a-z0-9]*)(-[a-z0-9]+)*$"
   scss/dollar-variable-colon-space-before: never
   scss/dollar-variable-pattern: "^(-?[a-z][a-z0-9]*)(-[a-z0-9]+)*$"