src: align worker and main thread code with embedder API

This addresses some long-standing TODOs by Joyee and me about
making the embedder API more powerful and us less reliant on
internal APIs for creating the main thread and Workers.

PR-URL: https://github.com/nodejs/node/pull/30467


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