[lua] Correctly call PropertiesIterator destructor using its __gc

This was detected by MSVC with its _ITERATOR_DEBUG_LEVEL=2 where
orphan iterators are detected when the map/container is destroyed.

src/app/script/properties_class.cpp

@@ -150,6 +150,12 @@ int Properties_pairs(lua_State* L)
   return 2;
 }
 
+int PropertiesIterator_gc(lua_State* L)
+{
+  get_obj<PropertiesIterator>(L, 1)->~PropertiesIterator();
+  return 0;
+}
+
 const luaL_Reg Properties_methods[] = {
   { "__len", Properties_len },
   { "__index", Properties_index },
@@ -159,6 +165,7 @@ const luaL_Reg Properties_methods[] = {
 };
 
 const luaL_Reg PropertiesIterator_methods[] = {
+  { "__gc", PropertiesIterator_gc },
   { nullptr, nullptr }
 };