worker: add mutex lock to MessagePort ctor

Automated tooling for race condition detection reports this as a
possible problem. It’s unlikely that something bad would happen
here (beyond maybe calling `TriggerAsync()` twice), but adding this
should be okay.

PR-URL: https://github.com/nodejs/node/pull/25911


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: James M Snell <jasnell@gmail.com>