Test CSS @layer, @property properties

[aeschli]

Refs: #147354 #157267

• Any OS @rzhao271
• Any OS @benibenj

Complexity: 2

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• The CSS language parser now supports the new @layer and @Property at rules
• See https://developer.mozilla.org/en-US/docs/Web/CSS/@layer, https://developer.mozilla.org/en-US/docs/Web/CSS/@property
• Verify that the validator accepts correct syntax but complains about syntax errors
• Verify that you get completions and hovers for rules inside the at rules