I can't find of a way to trigger this codepath, and there is for sure no
correpsonding WPT test, but let's fix this up for whenever it does
become relevant.
Which will have proper handling of the exection context when performing
a microtask checkpoint. This fixes an assertion to be added in the next
commit where perform_a_microtask_check is invoked on a non-empty
execution context stack.
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
The main motivation behind this is to remove JS specifics of the Realm
from the implementation of the Heap.
As a side effect of this change, this is a bit nicer to read than the
previous approach, and in my opinion, also makes it a little more clear
that this method is specific to a JavaScript Realm.
