path: replace assertPath() with validator

The path module's assertPath() does exactly what the
validateString() validator does, so this commit updates
path to use validateString() instead. A couple drive by
updates to validateString() outside of assertPath() are
also included.

PR-URL: https://github.com/nodejs/node/pull/24840


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>