[Tasks] Add pre-task update event (#2512)

This adds the player EVENT_TASK_BEFORE_UPDATE event which will allow
quests to prevent a source controlled task update by returning non-zero.

zone/event_codes.h

@@ -103,6 +103,7 @@ typedef enum {
 	EVENT_MERCHANT_BUY,
 	EVENT_MERCHANT_SELL,
 	EVENT_INSPECT,
+	EVENT_TASK_BEFORE_UPDATE,
 	_LargestEventID
 } QuestEventID;