LibWeb: Serialize transition as a coordinating list shorthand

Author: https://github.com/Calme1709
Commit: bb7d5747e7
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/6508
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/CSS/StyleValues/ShorthandStyleValue.cpp

@@ -838,6 +838,8 @@ String ShorthandStyleValue::to_string(SerializationMode mode) const
 
         return builder.to_string_without_validation();
     }
+    case PropertyID::Transition:
+        return coordinating_value_list_shorthand_to_string("all"sv);
     case PropertyID::WhiteSpace: {
         auto white_space_collapse_property = longhand(PropertyID::WhiteSpaceCollapse);
         auto text_wrap_mode_property = longhand(PropertyID::TextWrapMode);