Check CSS styling in CI

[ndr-ds]

Maybe this is less relevant, but might be a good idea since it's a repository with UI code and .css files.
We can run npx stylelint "**/*.css" from CI.
You need to create a config file for this, but something like a .stylelintrc.json file under the extension directory, with this:

{
  "extends": "stylelint-config-standard",
}

Might be a good enough starting point.