child_process: add ChildProcess 'spawn' event

The new event signals that the subprocess has spawned successfully and
no 'error' event will be emitted from failing to spawn.

Fixes: https://github.com/nodejs/node/issues/35288
PR-URL: https://github.com/nodejs/node/pull/35369


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>