src: remove AsyncRequest

Remove `AsyncRequest` from the source code, and replace its
usage with threadsafe `SetImmediate()` calls. This has the
advantage of being able to pass in any function, rather than
one that is defined when the `AsyncRequest` is “installed”.

This necessitates two changes:

- The stopping flag (which was only used in one case and ignored
  in the other) is now a direct member of the `Environment` class.
- Workers no longer have their own libuv handles, requiring
  manual management of their libuv ref count.

As a drive-by fix, the `can_call_into_js` variable was turned
into an atomic variable. While there have been no bug reports,
the flag is set from `Stop(env)` calls, which are supposed to
be possible from any thread.

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>