Convert SASS-style inline comments to CSS-style multiline comments. (#11733)

Following on from [the conversion of SASS ->
CSS](#11677), this PR converts
the SASS style inline comments to CSS style multiline comments.

Since this was the only reason we were using `postcss-scss` parser, it
too has been removed.

---

I used this script to achieve the conversion:

```sh
#!/bin/bash

# Write a temporary postcss config for postcss to work
cat <<EOF > postcss.config.js
const spaceAtEndRegex = /\s$/;
const postCssInlineToMultilineComment = {
  postcssPlugin: 'postcss-inline-to-multiline-comment',
  Comment(node) {
    if (node.raws.inline && (!node.raws.right || !spaceAtEndRegex.test(node.raws.right))) {
      if (!node.raws.right) {
        node.raws.right = '';
      }
      node.raws.right += ' ';
    }
  },
}
module.exports = {
  // Need to understand SCSS inline comment syntax
  parser: 'postcss-scss',
  plugins: [
    postCssInlineToMultilineComment,
  ],
};
EOF

npx postcss-cli --yes postcss-cli -r --no-map --config="`pwd`" polaris-react/**/*.css

rm -f postcss.config.js  # Clean up temporary files
```

Then I confirmed it was successful with this script:

```sh
if [ -z "$1" ]; then echo "A filename is required:\n> $0 ./no-inline.css"; exit 1; fi;

# Write a temporary postcss config for cssnano to work
cat <<EOF > postcss.config.js
module.exports = {
  plugins: [
    require('cssnano')({
      preset: 'lite',
    }),
  ],
};
EOF

yarn workspace @shopify/polaris add cssnano-preset-lite
yarn
yarn build --filter="@shopify/polaris"
cp polaris-react/build/esm/styles.css "$1" # Move out of ignored directories
yarn prettier -w "$1" # Pre-format to normalize whitespace
npx postcss-cli --yes postcss-cli -r --no-map --config="`pwd`" "$1" # cssnano removes more whitespace prettier doesn't
yarn prettier -w "$1" # Format again to get back into un-minified version
rm postcss.config.js # Clean up temporary files
yarn workspace @shopify/polaris remove cssnano-preset-lite
```

Run like this:

```sh
git checkout multiline-comments
./go.sh ./no-inline.css
git checkout main
./go.sh ./inline.css
git checkout -
git diff --histogram --no-index inline.css no-inline.css
# Should be an empty diff
```

Author: jesstelford

.changeset/brown-penguins-walk.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+Convert SASS-style inline comments to CSS-style multiline comments.

polaris-react/config/rollup/plugin-styles.js

@@ -2,7 +2,6 @@ const path = require('path');
 
 const {createFilter} = require('@rollup/pluginutils');
 const postcss = require('postcss');
-const postcssScss = require('postcss-scss');
 const cssModules = require('postcss-modules');
 
 module.exports.styles = function styles({
@@ -145,7 +144,7 @@ module.exports.styles = function styles({
       }
 
       const postCssOutput = await styleProcessor
-        .process(source, {parser: postcssScss, from: id})
+        .process(source, {from: id})
         .then((result) => ({
           css: result.css,
           tokens: result.messages.find(({plugin, type}) => {