support for @layer

[mayank99]

currently, when using cascade layers, the csstree-validator extension shows this error:

[CSSTree] Unknown at-rule `@layer`

@layer has been supported in all modern browsers for about a year now, so it would be nice to have support for it.

i understand that it can be difficult to maintain compatibility with every new feature and i'm happy to help in any way. i also think that it would be nice to have some kind of "allowlist" to prevent errors on such new syntaxes.

[skyclouds2001]

seems that mdn-data has @layer data yet

https://github.com/mdn/data/blob/31d4ebe3cdf79c3a13c8c601c18bcc90c0cbb0fe/css/at-rules.json#L324-L335