style: Hook up basic @layer rule parsing

Disabled, and of course doing nothing for now still, but this is another
piece that is useful to get reviewed separately.

Don't allow layers to be interleaved with @import / @namespace rules as
per https://github.com/w3c/csswg-drafts/issues/6522.

Differential Revision: https://phabricator.services.mozilla.com/D124229

components/style/stylesheets/mod.rs

@@ -372,8 +372,7 @@ impl CssRule {
             // CssRule::Charset(..) => State::Start,
             CssRule::Import(..) => State::Imports,
             CssRule::Namespace(..) => State::Namespaces,
-            // TODO(emilio): We'll need something here for non-block layer
-            // rules.
+            // TODO(emilio): Do we need something for EarlyLayers?
             _ => State::Body,
         }
     }