util: make inspect aware of RegExp subclasses and null prototype

This adds support for inspect to distinguish regular expression
subclasses and ones with null prototype from "normal" regular
expressions.

PR-URL: https://github.com/nodejs/node/pull/25192


Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>