The inert temporary documents used for fragment parsing will never
become "fully active" and so any tasks associated with them will never
run. To avoid memory leaks, we now simply ignore any attempts to enqueue
tasks associated with such documents.
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
