Replace DirectionalLight3D's use_in_sky_only with sky_mode enum

3 options are available:

- Light and Sky (default)
- Light Only (new)
- Sky Only (equivalent to `use_in_sky_only = true`)

Co-authored by: clayjohn <claynjohn@gmail.com>

servers/rendering/renderer_scene_cull.h

@@ -689,7 +689,7 @@ public:
 			float spot_angle;
 			float spot_attenuation;
 			bool has_shadow;
-			bool sky_only;
+			RS::LightDirectionalSkyMode sky_mode;
 		};
 
 		Vector<LightCache> light_cache;