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
