worker: correct (de)initialization order

• Initialize thread_exit_async_ only once the thread has been started. This is done since it is only triggered from the thread when it is exiting.
• Move the final uv_run to the Worker destructor. This makes sure that it is always run, regardless of whether the thread is actually started or not.
• Always dispose the Isolate before cleaning up the libuv event loop. This now matches the reverse order of initialization.

Fixes: https://github.com/nodejs/node/issues/22736

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

/cc @nodejs/workers