src: split per-process initialization and teardown routines

This patch makes it possible to instantiate NodeMainInstance in
a separate target and use it to e.g. create snapshot.

PR-URL: https://github.com/nodejs/node/pull/27276


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>