lib: created isValidCallback helper

check for callback function is moved to a separate function.
This piece of code is being shared by other entities as well.

PR-URL: https://github.com/nodejs/node/pull/32665


Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>