Support multiple UFOs at once

[RickyDaMa]

Changes the CLI to support taking multiple inputs. Incompatible with -o/--output

Happy to adjust implementation/testing as required. If there's any bad formatting let me know, I didn't see a strong standard defined anywhere and my editor's formatter was ready to eviscerate the whole file

I've tried to keep the program acting as similarly as possible - e.g. exiting with code 2 if no valid paths were provided - but also permissive, so the whole program doesn't bomb out if one input path of many is bad

Also, as far as I can see we could mark Python 3.12 as officially supported (I used it while developing/testing these changes), I can update the README & CI if you'd like :)

Finally, please consider making the repository more approachable to new contributors: https://matklad.github.io/2022/10/24/actions-permissions.html