src: associate is_main_thread() with worker_context()

In our codebase, the assumption generally is that `!is_main_thread()`
means that the current Environment belongs to a Node.js Worker thread.

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>