Implemented NPC special ability 40 'NPC_CHASE_DISTANCE'

Param 0: Sets max distance you need to be away from an npc for it to chase you.
Param 1: Sets min distance you need to be from npc for it to chase you.
Usage: Ideally used with ranged attack npcs / casters who you DO NOT WANT
to chase you unless you get too close or too far or out of sight.

zone/common.h

@@ -127,7 +127,8 @@ enum {
 	FLEE_PERCENT = 37,
 	ALLOW_BENEFICIAL = 38,
 	DISABLE_MELEE = 39,
-	MAX_SPECIAL_ATTACK = 40
+	NPC_CHASE_DISTANCE = 40,
+	MAX_SPECIAL_ATTACK = 41
 	
 };