DEV: Introduce stylelint (#29852)

Stylelint is a css linter: https://stylelint.io/

As part of this change we have added two javascript scripts:

```
pnpm lint:css
pnpm lint:css:fix
```

Look at `.vscode/settings.json.sample` and `.vscode/extensions.json` for
configuration in VSCode.

---------

Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

.github/workflows/linting.yml

@@ -74,6 +74,10 @@ jobs:
         if: ${{ !cancelled() }}
         run: pnpm lint:hbs
 
+      - name: Stylelint
+        if: ${{ !cancelled() }}
+        run: pnpm lint:css
+
       - name: English locale lint (core)
         if: ${{ !cancelled() }}
         run: bundle exec ruby script/i18n_lint.rb "config/**/locales/{client,server}.en.yml"