fix(input): fix URL validation being too strict

Background:
Prior to ffb6b2f, there was a bug in `URL_REGEXP`, trying to match the hostname as `\S+` (meaning
any non-space character). This resulted in never actually validating the structure of the URL (e.g.
segments such as port, path, query, fragment).
Then ffb6b2f and subsequently e4bb838 fixed that bug, but revealed `URL_REGEXP`'s "strictness" wrt
certain parts of the URL.

Since browsers are too lenient when it comes to URL validation anyway, it doesn't make sense for
Angular to be much stricter, so this commit relaxes the "strictness" of `URL_REGEXP`, focusing more
on the general structure, than on the specific characters allowed in each segment.

Note 1: `URL_REGEXP` still seems to be stricter than browsers in some cases.
Note 2: Browsers don't always agree on what is a valid URL and what isn't.

Fixes #13528

Closes #13544