The promise job's fulfillment / rejection handlers may push an execution
context onto the VM, which will throw an internal error if our ad-hoc
call stack size limit has been reached. Thus, we cannot blindly VERIFY
that the result of invoking these handlers is non-abrupt.
This patch will propagate any internal error forward, and retains the
condition that any other error type is not thrown.
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