worker: move `receiving_messages_` field to `MessagePort`

This is a property of the native object associated with the
`MessagePort`, not something that should be set on the
conceptual `MessagePort` that may be transferred around.

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


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