We weren't doing this before either, but through a slightly sneaky
mechanism: we had overridden Cell::initialize() in
ShadowRealmGlobalScope as a no-op.
Instead of that, do the same thing Window and Worker globals do and
make all of the globals that inherit UniversalGlobalScopeMixin opt
out of WEB_SET_PROTOTYPE_FOR_INTERFACE in EventTarget::initialize().
All global objects current need to be event targets so that they
can have events dispatched to them. This allows for removing of
verify_cast for these global objects.
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
