Lua : add simple userdata type

std/lua/Boot.hx

@@ -110,6 +110,8 @@ class Boot {
 				return Lua.type(o) == "string";
 			case Thread:
 				return Lua.type(o) == "thread";
+			case UserData:
+				return Lua.type(o) == "userdata";
 			case Array:
 				return Lua.type(o) == "table"
 					&& untyped o.__enum__ == null

std/lua/UserData.hx

@@ -0,0 +1,2 @@
+package lua;
+class UserData {}