src: mark options parsers as const

These do not change their contents after being constructed.

PR-URL: https://github.com/nodejs/node/pull/25065


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>