process: emit unhandled warning immediately

PR-URL: https://github.com/nodejs/node/pull/24632
Fixes: https://github.com/nodejs/node/issues/24209


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>