Add jpegxl support

[Luke Warlow]

With Safari 17 coming with native support for JPEGXL images it'd be good for this tool to support the format.

I believe sharp has an option for JXL https://sharp.pixelplumbing.com/api-output#jxl but the documentation suggests it not a standard function.

Apologies if this is already supported and I've just missed it.

[Ben McCann]

https://caniuse.com/jpegxl