src: clean up WHATWG WG parser

* reduce indentation
* refactor URL inlined methods
* prefer templates over macros
* do not export ARG_* flags in url binding

PR-URL: https://github.com/nodejs/node/pull/12251


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>