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 b34f05e, that is the case.

Fixes: nodejs#22940

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: