add `URL::join(string_view relative)` ?

[kvtb]

It would be very convenient to have URL::join function to construct a new URL from an existing URL and a relative path (for example one from Location server header on 302-redirect).

Despite simplicity, implementation has some corner cases, including security sensitive ones, so it is better to have it implemented in a library than to left is an an exercise to a library user.

[afrind]

Happy to take a PR if you are feeling ambitious

[afrind]

Would still accept a PR for this but otherwise it's nonurgent.