This is a bit of a chonkier commit as it results in both:
clean_up_after_running_callback and prepare_to_run_callback being
changed to accept a realm instead of an environment settings object,
which has a bunch of fallout, particuarly for IDL abstract operations.
In the cases where spec authors have us directly interact with promises
in a task source context, we need to prepare the backup settings object
stack as well as push an actual execution context to the JS VM.
We've peppered this workaround around the code base as needed in a few
different ways. This adds a helper class to perform this workaround in
order to simplify doing so, and ensure cleanup occurs in a RAII fashion.
This also makes it easier to grep for places where this workaround is
employed.
