src: add a threadsafe variant of SetImmediate()

Add a variant of `SetImmediate()` that can be called from any thread.
This allows removing the `AsyncRequest` abstraction and replaces it
with a more generic mechanism.

Backport-PR-URL: https://github.com/nodejs/node/pull/32301
PR-URL: https://github.com/nodejs/node/pull/31386
Refs: https://github.com/openjs-foundation/summit/pull/240


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>