add RGFW examples

[ColleagueRiley]

This adds RGFW examples to nuklear including,

rgfw_opengl2, rgfw_opengl3, and rgfw_opengl4 as well as an rgfw example for rawfb.

(I think it would be better if Nuklear separated the platform API from the graphics API, similar to Dear ImGUI)

[RobLoach]

This is great, thanks a lot. Is there a way to re-use some of the same code across the different implementations? Could save some maintenance time. Perhaps have a parent nuklear_rgfw.h, and then nuklear_rgfw_*.h for each individual implementation?

[ColleagueRiley]

@RobLoach
This is a problem for each platform as far as I can tell.

Dear ImGUI handles it with
imgui_opengl.h and imgui_.h

Where the graphics API platform can work with any windowing API platform. Assuming the platform has support for that graphics API.