querystring, url: handle repeated sep in search

* update state machine in parse
  * repeated sep should be adjusted
  * `&=&=` should be `{ '': [ '', '' ] }`
* add test cases for querystring and URLSearchParams

Fixes: https://github.com/nodejs/node/issues/10454
PR-URL: https://github.com/nodejs/node/pull/10967


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>