worker: use special message as MessagePort close command

(The first commit is cleanup from https://github.com/nodejs/node/pull/27294, with which this is going to have conflicts anyway…)

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

Checklist

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