src: split ownsProcessState off isMainThread

Embedders may want to control whether a Node.js instance
controls the current process, similar to what we currently
have with `Worker`s.

Previously, the `isMainThread` flag had a bit of a double usage,
both for indicating whether we are (not) running a Worker and
whether we can modify per-process state.

PR-URL: https://github.com/nodejs/node/pull/25881


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>