src: disallow JS execution inside FreeEnvironment

This addresses a TODO comment, and aligns the behavior between
worker threads and the main thread.

The primary motivation for this change is to more strictly enforce
the invariant that no JS runs after the `'exit'` event is emitted.

PR-URL: https://github.com/nodejs/node/pull/33874


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>