RGBDS syntax updates (#905)

New MACRO and DEF syntax

constants/scene_constants.asm

@@ -2,13 +2,13 @@
 ; Each scene_script and coord_event is associated with a current scene ID.
 
 ; Scene variables default to 0.
-SCENE_DEFAULT  EQU 0
+DEF SCENE_DEFAULT  EQU 0
 ; Often a map will have a one-time default event for scene 0, and switch to a
 ; do-nothing scene 1 when the event finishes.
-SCENE_FINISHED EQU 1
+DEF SCENE_FINISHED EQU 1
 ; A coord_event for scene -1 will always activate, regardless of the map's
 ; scene variable value.
-SCENE_ALWAYS   EQU -1
+DEF SCENE_ALWAYS   EQU -1
 
 ; wPokecenter2FSceneID
 	const_def 1