AK: Add a query string component to URL

It's missing query string parsing from new URLs, but you can set the
query string programmatically, and it will be part of the URL when
serialized through to_string().

AK/URL.cpp

@@ -144,6 +144,10 @@ String URL::to_string() const
         }
     }
     builder.append(m_path);
+    if (!m_query.is_empty()) {
+        builder.append('?');
+        builder.append(m_query);
+    }
     return builder.to_string();
 }