[lib][web][native] Refactor urlPrefixSelector

Summary: issue: https://linear.app/comm/issue/ENG-5001/refactor-urlprefixselector

Test Plan: Tested that the selector works by calling useSelector(urlPrefixSelector('256')); and printing the result. Tested the openSocketSelector by running the app and checking that it is possible to send messages

Reviewers: michal, kamil, ginsu

Reviewed By: kamil

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9681