Add example of GDExtension written pure C

[vnen]

This is the same code as the C++ example, but written without any bindings using the GDExtension API directly. It is somewhat long because it needs to touch a lot of aspects and create boilerplate that would normally come from the bindings.

I also want to point out that the GDExtension API is not meant to be used directly like this, so it is more of a reference for people who wants to create their own bindings.

[vnen]

BTW, thanks for all the reviews. I did kind of forget this during weekend (on purpose to rest) but in the next few days I'll address all the issues pointed out.