url: using util._extend for improving profermace

`Object.assign` is much slower than `util._extend` according to
the refs. This change is to convert the `Object.assign` to use
`util._extend` in url module for improving profermance.

PR-URL: https://github.com/nodejs/node/pull/16081
Refs: https://github.com/nodejs/CTC/issues/62


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>