remove submodules, add full physfs

[konsumer]

I had issues with miniphysfs building on mac, and noticed that the compiled size is roughly the same, depending on the options you use (for example disabling all but zip.) This PR shows an easy-to-build & simple alternative setup, using regular physfs, that pulls raylib & physfs from git (no submodules, just cmake config.)

I find this kind of structure much more versatile, easier to use/setup, but I think for your lib it probly needs a bit more work to make it nicer for users (I'm not really exposing the lib correctly, just static building the demos.)

WARNING: This is not really a drop-in replacement, as I wasn't quite sure how to structure the project to work well as a dep of user's projects. This is more of an example project that builds all the examples.

[konsumer]

If instructions were simply "put this .h file in your project, and link cmake like this..." it might be simple enough, but you probly know a better way to set it all up. I think in my own project I will probly just drop in the raylib-physfs header, and use my current cmake that pulls raylib & physfs.