inspector: mark as stable

The module has been there for a while,
evidence of usage in the field exists,
and we have good test coverage for it.
Last API change has been 18 months ago.

Refs: #37440 (comment)

PR-URL: #37748
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/inspector.md

@@ -2,7 +2,7 @@
 
 <!--introduced_in=v8.0.0-->
 
-> Stability: 1 - Experimental
+> Stability: 2 - Stable
 
 <!-- source_link=lib/inspector.js -->