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>

plugins/spoiler-alert/assets/stylesheets/discourse_spoiler_alert.scss

@@ -1,6 +1,6 @@
 .spoiled {
   cursor: auto;
-  -webkit-transform: translateZ(0); // Safari jitter fix
+  transform: translateZ(0); // Safari jitter fix
 
   .lightbox .meta {
     display: none;