util: improve function inspection

This commit contains the following changes:

1) Add null prototype support for functions.
2) Safely detect async and generator functions.
3) Mark anonymous functions as such instead of just leaving out the
   name.

PR-URL: https://github.com/nodejs/node/pull/27227


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: John-David Dalton <john.david.dalton@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>