process: add optional code to warnings + type checking

Add the ability to assign an optional code to process warnings +
add additional type checking to ensure that names and codes can
only be strings.

PR-URL: https://github.com/nodejs/node/pull/10116


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Michal Zasso <targos@protonmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>