While the code that did this referred to the HTML spec, other browsers
appear not to have this behavior when parsing XML, and it breaks a WPT
subtest.
This change does not appear to break any tests, and fixes 1 WPT subtest.
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