If we've already fired off an error, calling stop_fetch() should
make no difference, other than stopping the Requests::Request.
Eventually, we'll probably want abort() and terminate() to
eventually stop the Requests::Request in an unobservable way.
This allows the Request to be cleaned up when it becomes inactive,
which in turn allows the GC to clean up the FetchController which is
indirectly captured by a root in the Request callbacks.
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
Now that the heap has no knowledge about a JavaScript realm and is
purely for managing the memory of the heap, it does not make sense
to name this function to say that it is a non-realm variant.
