imgui ffi bindings

Hey chicken-users!

I extracted the imgui bindings from my project. It's not ready to be
hatched as an egg, more like in oocyte state right now, but if you want to
give it a try, here's the repo:

https://forgejo.rolando.cl/cpm/chicken-imgui

The egg only builds on macOS right now, haven't had the chance to add the
right cond-expand for linux/windows. I also don't know exactly how to deal
with custom csc flags without a custom build script (which I was trying to
avoid).

There's a readme in the repo with instructions. I also included a minimal
sdl & gl binding (needed for the imgui backend). If you have any questions,
let me know! If you have improvements, feel free to send me a patch!
I think there's an opportunity to get rid of the sdl & gl bindings by
creating a higher level abstraction, but haven't thought too much about it
- open to suggestions!

Enjoy!
Rolando