src: WHATWG URL C++ parser cleanup

- Clarify port state
- Remove scheme flag
- Clarify URL_FLAG_TERMINATED

PR-URL: https://github.com/nodejs/node/pull/11917


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