Relocate RGB keycode processing (#7508)

* Move rgb keycode logic to process_keycode

* Fixes for rgb matrix

* Fixes for mxss

* Fix inc/dec logic, add comments

* Fix return RAINBOW_SWIRL logic

* stop external use of rgb helper functions

* merge fix

* Fix 'defined but not used' when all animations are disabled

quantum/quantum.h

@@ -137,6 +137,10 @@ extern layer_state_t layer_state;
 #    include "process_magic.h"
 #endif
 
+#if defined(RGBLIGHT_ENABLE) || defined(RGB_MATRIX_ENABLE)
+#    include "process_rgb.h"
+#endif
+
 #ifdef HD44780_ENABLE
 #    include "hd44780.h"
 #endif