Add and apply function-calculation-no-interpolation rule

This stylelint-scss rule is useful because we were inconsistent when
using calc(); sometimes we added interpolation to Sass variables, and
sometimes we didn't. The reason why we originally added interpolation
was that it was necessary until we migrated to Dart Sass in commit
d54971e53. Since then, we can omit the interpolation, which is also what
the Sass documentation recommends [1].

[1] https://sass-lang.com/documentation/values/calculations/

.stylelintrc.yml

@@ -66,6 +66,7 @@ rules:
   scss/at-mixin-pattern: "^(-?[a-z][a-z0-9]*)(-[a-z0-9]+)*$"
   scss/dollar-variable-colon-space-before: never
   scss/dollar-variable-pattern: "^(-?[a-z][a-z0-9]*)(-[a-z0-9]+)*$"
+  scss/function-calculation-no-interpolation: true
   scss/load-no-partial-leading-underscore: true
   scss/load-partial-extension: never
   scss/no-unused-private-members: true