Hooked up raylib as a library in the examples build so that if a change
is made in raylib then running `zig build` in the examples directory
will automatically rebuild raylib.

The main pieces to this are the build.zig.zon file which allows the
examples build to see the parent raylib directory.  Then we can add
raylib as a typical dependency and grab the "raylib" artifact to link
to. We also no longer need to add include paths since raylib's build.zig
installs them for us.