This adds a method to shut down the toggle queue so that it doesn't
accept any more toggles. This is intended to stop toggles happening that
are queued from other threads while the GjsContext dispose sequence has
already entered its final garbage collection.

We clear pending toggles and shut down the toggle queue before chaining
up in GjsContext's dispose function. Sending out dispose notifications
causes all GjsMaybeOwned instances to unroot themselves, which means that
toggles don't have much meaning after that point.

Closes #26.