worker: mark as stable

This feature is not expected to receive breaking changes to its API
and is used in real-world applications.

As discussed at the last collaborator summit (Berlin May 2019),
the `worker_threads` module can be considered stable once our
Web Messaging implementation is compatible with Web Platform Tests.
As of b34f05ec, that is the case.

Fixes: https://github.com/nodejs/node/issues/22940

PR-URL: https://github.com/nodejs/node/pull/29512


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>