LibWeb: Move media-query-list serialization code to MediaQuery.{h,cpp}

It's not a complicated algorithm, but having it in one place instead of
2, and with spec comments, is nice. :^)

Userland/Libraries/LibWeb/CSS/MediaQuery.h

@@ -93,6 +93,8 @@ private:
     bool m_matches { false };
 };
 
+String serialize_a_media_query_list(NonnullRefPtrVector<MediaQuery> const&);
+
 }
 
 namespace AK {