JXL Support

This release has two changes. The main one is #13 which adds support for JXL images. This isn't a format I was aware of but @oscar-rc1 was kind enough to create a PR to add it.

Behind the scenes there was also some minor refactoring which may be expanded later. The interface should stay the same but I may work on restructuring things further in the future. I doubt there will be any forward facing interface changes though.