Invalidation for adopted style sheets was broken because we had an
assumption that "active" style sheet is always attached to
StyleSheetList which is not true for adopted style sheets. This change
addresses that by keeping track of all documents/shadow roots that own
a style sheet and notifying them about invalidation instead of going
through the StyleSheetList.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
It's possible for absolutizing a selector to return an invalid selector
(eg, it could cause `:has()` inside `:has()`) so we need to be able to
express that.
The main motivation behind this is to remove JS specifics of the Realm
from the implementation of the Heap.
As a side effect of this change, this is a bit nicer to read than the
previous approach, and in my opinion, also makes it a little more clear
that this method is specific to a JavaScript Realm.
