embedding: provide hook for custom process.exit() behaviour

Embedders may not want to terminate the process when `process.exit()`
is called. This provides a hook for more flexible handling of that
situation.

Refs: https://github.com/nodejs/node/pull/30467#issuecomment-603689644

PR-URL: https://github.com/nodejs/node/pull/32531


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