This begins implementation on form-associated custom elements.
This fixes a few WPT tests which I'm importing.
Co-authored-by: Sam Atkins <sam@ladybird.org>
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
