worker: make terminate() resolve for unref’ed Workers

Once `worker.terminate()` is called, the Worker instance will be
destroyed as soon as possible anyway, so in order to make
the Promise returned by `worker.terminate()` resolve always,
it should be okay to just call `.ref()` on it and keep the main
event loop alive temporarily.

PR-URL: https://github.com/nodejs/node/pull/29484


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>