Test 'CSS Language Features extension' in web

[aeschli]

#97701

• anyOS: @Tyriar Chrome
• macOS: @isidorn Safari
• anyOS @joaomoreno Firefox

Complexity: 3

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• From the VSCode sources, run yarn web and open http://localhost:8080 in the browser
• open the 'css' file and verify that rich css/less/scss language features work
  See https://code.visualstudio.com/docs/languages/css for more info
• validation
• code completion
• code completion for paths (e.g. background: url(^))
• hovers, for properties, and selectors
• color decorators
• css settings
• links
• custom data from settings (see https://github.com/Microsoft/vscode-css-languageservice/blob/master/docs/customData.md)

[isidorn]

@aeschli I am sort of blocked for testing this on Safari, since nothing seems to work
https://github.com/microsoft/vscode/issues/99049