util: improve internal `isError()` validation

The current internal isError function checked the toString value
instead of using the more precise `util.types.isNativeError()` check.
The `instanceof` check is not removed due to possible errors that
are not native but still an instance of Error.

PR-URL: https://github.com/nodejs/node/pull/24746


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>