This overrides the JS host hooks to follow the spec for queuing
promises, making/calling job callbacks, unhandled promise rejection
handling and FinalizationRegistry queuing.
This also allows us to drop the on_call_stack_emptied hook in
Document::interpreter().
We now invoke DOM timer callbacks via HTML tasks. This brings callback
sequencing closer to the spec, although there are still many
imperfections in this area.
This patch attaches a HTML::EventLoop to the main thread JS::VM used
for JavaScript bindings in the web engine.
The goal here is to model the various task scheduling mechanisms of the
HTML specification.
