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/malloc_size_of/lib.rs

@@ -748,6 +748,9 @@ where
             Component::Slotted(ref selector) | Component::Host(Some(ref selector)) => {
                 selector.size_of(ops)
             },
+            Component::Is(ref list) | Component::Where(ref list) => {
+                list.size_of(ops)
+            },
             Component::PseudoElement(ref pseudo) => (*pseudo).size_of(ops),
             Component::Combinator(..) |
             Component::ExplicitAnyNamespace |