src: track no of active JS signal handlers

This makes it possible to tell whether a signal is being tracked in JS.

PR-URL: https://github.com/nodejs/node/pull/30229


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>