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 b34f05ecf2, 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>

doc/api/worker_threads.md

@@ -2,7 +2,7 @@
 
 <!--introduced_in=v10.5.0-->
 
-> Stability: 1 - Experimental
+> Stability: 2 - Stable
 
 The `worker_threads` module enables the use of threads that execute JavaScript
 in parallel. To access it: