[Commands] Add #clearxtargets Command (#3833)

# Perl
- Add `$client->ClearXTargets()`.

# Lua
- Add `client:ClearXTargets()`.

# Notes
- Add `#clearxtargets` command for players.
- Allows operators/players to clear their XTargets if something get stuck on it.

zone/client.h

@@ -1528,6 +1528,7 @@ public:
 	void JoinGroupXTargets(Group *g);
 	void LeaveGroupXTargets(Group *g);
 	void LeaveRaidXTargets(Raid *r);
+	void ClearXTargets();
 	bool GroupFollow(Client* inviter);
 	inline bool  GetRunMode() const { return runmode; }