Add placeholder constants for ObjectStructs offsets.

Note that OBJECT_LENGTH is not associated with ObjectStructs (it's isntead used as the length of a MapObjects struct).

constants/map_constants.asm

@@ -1771,10 +1771,19 @@ GATE EQU 6
 DUNGEON EQU 7
 
 ; object struct
+OBJECT_00 EQU $0
+OBJECT_01 EQU $1
 OBJECT_SPRITE EQU $2
+OBJECT_03 EQU $3
+OBJECT_04 EQU $4
 OBJECT_FLAGS EQU $5
 OBJECT_PALETTE EQU $6
+OBJECT_07 EQU $7
+OBJECT_08 EQU $8
+OBJECT_09 EQU $9
 OBJECT_STEP_DURATION EQU $a
+OBJECT_11 EQU $b
+OBJECT_12 EQU $c
 OBJECT_FACING EQU $d
 OBJECT_STANDING_TILE EQU $e
 OBJECT_NEXT_TILE EQU $f
@@ -1782,8 +1791,18 @@ OBJECT_MAP_X EQU $10
 OBJECT_MAP_Y EQU $11
 OBJECT_NEXT_MAP_X EQU $12
 OBJECT_NEXT_MAP_Y EQU $13
+OBJECT_20 EQU $14
+OBJECT_21 EQU $15
+OBJECT_22 EQU $16
 OBJECT_SPRITE_X EQU $17
 OBJECT_SPRITE_Y EQU $18
 OBJECT_SPRITE_X_OFFSET EQU $19
 OBJECT_SPRITE_Y_OFFSET EQU $1a
+OBJECT_27 EQU $1b
+OBJECT_28 EQU $1c
+OBJECT_29 EQU $1d
+OBJECT_30 EQU $1e
+OBJECT_31 EQU $1f
+OBJECT_32 EQU $20
+; 33-39 are not used