support for `@layer`

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.