[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/lua_client.h

@@ -488,6 +488,7 @@ public:
 	void RemoveRadiantCrystals(uint32 amount);
 	void SummonItemIntoInventory(luabind::object item_table);
 	bool HasItemOnCorpse(uint32 item_id);
+	void ClearXTargets();
 
 	void ApplySpell(int spell_id);
 	void ApplySpell(int spell_id, int duration);