Create models from ShEx files

[NoelDeMartin]

I haven't found the need to use ShEx files myself, but I've seen a couple of people mentioning it so I decided to create this issue to gather interest.

I'm not familiar with the format, but looking at it I think it shouldn't be too difficult to implement, so if anyone is interested in that please let me know here :). Not sure when I'll implement it though, but if more people is interested I guess I'll prioritize it (and if nobody is, maybe I don't xD).

h/t to LDO and shex-codegen for their existing work on this, I'll probably look at their implementation before tackling mine.