Add WebP support to ImageSize

[silby]

My code feels a bit messy and I've tested it with like precisely one WebP image per variant (lossy, lossless, and extended). I found myself having/wanting to combine attoparsec (to get Alternative and backtracking) and Data.Binary.Get (to get numbers); maybe there's a less awkward looking way to do this.

This is an API change because the constructors of ImageType are public.

Closes #9561

[tarleb]

This code is quite low-level, which makes me think that it would better be placed in a different package. Would extending JuicyPixels be an option?

[silby]

It doesn’t really make sense to drive-by contribute this code to JuicyPixels. It’s hardly the foundation for a full WebP codec. If one were to create a library that just extracts the dimensions of various image sizes it would make sense there but I have no particular interest in doing that. Bitwise operators aside, it’s no more low-level than emfSize and pdfSize. 

[jgm]

I think this is fine; in the past we used all kinds of low-level code to find image sizes; JuicyPixels allowed us to remove a lot of this low-level stuff, but if we still need it for webp, okay.