[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/embparser.cpp

@@ -160,6 +160,7 @@ const char *QuestEventSubroutines[_LargestEventID] = {
 	"EVENT_MERCHANT_BUY",
 	"EVENT_MERCHANT_SELL",
 	"EVENT_INSPECT",
+	"EVENT_TASK_BEFORE_UPDATE",
 };
 
 PerlembParser::PerlembParser() : perl(nullptr)
@@ -1465,6 +1466,7 @@ void PerlembParser::ExportEventVariables(
 		}
 
 		case EVENT_TASK_COMPLETE:
+		case EVENT_TASK_BEFORE_UPDATE:
 		case EVENT_TASK_UPDATE: {
 			Seperator sep(data);
 			ExportVar(package_name.c_str(), "donecount", sep.arg[0]);