url: use a class for WHATWG url[context]

The object is used as a structure, not as a map, which `StorageObject`
was designed for.

PR-URL: https://github.com/nodejs/node/pull/11930


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>