Autodetect gemini URLs and allow clicking on them

[lokesh-krishna]

Gemini is a new internet protocol about which you can learn more at gemini.circumlunar.space.

Presently, kitty auto-detects URLs for a few internet protocols. It would be really nice if it would also autodetect gemini URLs (which are of the pattern gemini://) and allow clicking on them to open a browser that handles those links.

Presently the only way to follow a gemini link is to manually highlight it and paste it into a browser.

[kovidgoyal]

Simply add it to url_prefixes in kitty.conf. The default value will add
gemini for the next release.