worker: move JoinThread() back into exit callback

de2c68c7 moved this call to
the destructor, under the assumption that that would essentially
be equivalent to running it as part of the callback since the
worker would be destroyed along with the callback.

However, the actual code in
`Environment::RunAndClearNativeImmediates()` comes with the subtlety
that testing whether a JS exception has been thrown
happens between the invocation of the callback and its destruction,
leaving a possible exception from `JoinThread()` potentially
unhandled (and unintentionally silenced through the `TryCatch`).

This affected exceptions thrown from the `'exit'` event of the
Worker, and made the `parallel/test-worker-message-type-unknown`
test flaky, as the invalid message was sometimes only received
during the Worker thread’s exit handler.

Fix this by moving the `JoinThread()` call back to where it was
before.

Refs: https://github.com/nodejs/node/pull/31386

Backport-PR-URL: https://github.com/nodejs/node/pull/32301
PR-URL: https://github.com/nodejs/node/pull/31468


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>