worker: use special message as MessagePort close command

When a `MessagePort` connected to another `MessagePort` closes, the
latter `MessagePort` will be closed as well. Until now, this is done
by testing whether the ports are still entangled after processing
messages. This leaves open a race condition window in which messages
sent just before the closure can be lost when timing is unfortunate.
(A description of the timing is in the test file.)

This can be addressed by using a special message instead, which is
the last message received by a `MessagePort`. This way, all previously
sent messages are processed first.

Fixes: https://github.com/nodejs/node/issues/22762
PR-URL: https://github.com/nodejs/node/pull/27705


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>