This will allow us to use the GC to manage the lifetime of objects
that are not C++ objects, such as Swift objects. In the future we
could expand this cursed FFI to other languages as well.
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
