This now takes a registry directly. As a temporary stop-gap, the old
method on Document is still here and calls the new one.
We do get a test regression:
"Creating an element in a cloned document and inserting into the
document must not enqueue a custom element upgrade reaction" in
"custom-elements/upgrading.html".
Possibly a spec bug?
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
