Add freedesktop URI prefix for --clone

As agreed on on Mastodon.

I've also aligned the second column the same way function arguments are aligned, because otherwise it looked weird with freedesktop: being much longer than the other entries. I can revert this if it's unwanted.

P.S. I was surprised these prefixes only work with --clone and not in the URI entry itself, i.e. I cannot launch Builder, tap "Clone Repository...", input "freedesktop:mesa/mesa" and have it do the right thing. Furthermore, it even visually indicates the URI is valid, lets you tap "Clone" and actually starts cloning via Git; the error only comes in later when Git says it cannot resolve the host. I can see how this can be confusing.