JSON: Templatize the JSON serialization code

This makes it possible to use something other than a StringBuilder for
serialization (and to produce something other than a String.) :^)

AK/JsonObject.cpp

@@ -3,28 +3,4 @@
 
 namespace AK {
 
-void JsonObject::serialize(StringBuilder& builder) const
-{
-    int index = 0;
-    builder.append('{');
-    for_each_member([&] (auto& key, auto& value) {
-        builder.append('"');
-        builder.append(key);
-        builder.append('"');
-        builder.append(':');
-        value.serialize(builder);
-        if (index != size() - 1)
-            builder.append(',');
-        ++index;
-    });
-    builder.append('}');
-}
-
-String JsonObject::serialized() const
-{
-    StringBuilder builder;
-    serialize(builder);
-    return builder.to_string();
-}
-
 }