url: refactor lib/internal/url.js

* set an identifier for the separator rather than using multiple
  instances of the same literal
* consistent arrow function body formatting

PR-URL: https://github.com/nodejs/node/pull/10912


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>