Documents that have never been associated with a browsing context will
never become "fully active" so we shouldn't schedule tasks in them since
they'll never run.
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
