worker: drain messages from internal message port

When the worker thread exits, drain the messages also from the internal
message port so that the call to 'kDispose' will occur only after all
the messages from the worker were processed in the parent, so stdio
messages from the worker will be successfully pushed to their target
streams in the parent.

PR-URL: https://github.com/nodejs/node/pull/24932
Fixes: https://github.com/nodejs/node/issues/24636


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>