style: Implement parsing / selector-matching for :is() and :where().

This implements the easy / straight-forward parts of the :where / :is
selectors.

The biggest missing piece is to handle properly invalidation when there
are combinators present inside the :where. That's the hard part of this,
actually.

But this is probably worth landing in the interim. This fixes some of
the visitors that were easy to fix.

Differential Revision: https://phabricator.services.mozilla.com/D70788

components/style/selector_map.rs

@@ -500,6 +500,15 @@ fn specific_bucket_for<'a>(component: &'a Component<SelectorImpl>) -> Bucket<'a>
         // match the slotted <span>.
         Component::Slotted(ref selector) => find_bucket(selector.iter()),
         Component::Host(Some(ref selector)) => find_bucket(selector.iter()),
+        Component::Is(ref list) | Component::Where(ref list) => {
+            if list.len() == 1 {
+                find_bucket(list[0].iter())
+            } else {
+                // TODO(emilio): We should handle the multiple element case by
+                // inserting multiple entries in the map.
+                Bucket::Universal
+            }
+        },
         _ => Bucket::Universal,
     }
 }