src: use uv_async_t for WeakRefs

Schedule a task on the main event loop, similar to what the HTML
spec recommends for browsers.

Alternative to https://github.com/nodejs/node/pull/30198

PR-URL: https://github.com/nodejs/node/pull/30616


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>