AK: JsonParser improvements

- Parsing invalid JSON no longer asserts
    Instead of asserting when coming across malformed JSON,
    JsonParser::parse now returns an Optional<JsonValue>.
- Disallow trailing commas in JSON objects and arrays
- No longer parse 'undefined', as that is a purely JS thing
- No longer allow non-whitespace after anything consumed by the initial
  parse() call. Examples of things that were valid and no longer are:
    - undefineddfz
    - {"foo": 1}abcd
    - [1,2,3]4
- JsonObject.for_each_member now iterates in original insertion order

Userland/gron.cpp

@@ -74,6 +74,7 @@ int main(int argc, char** argv)
 
     auto file_contents = file->read_all();
     auto json = JsonValue::from_string(file_contents);
+    ASSERT(json.has_value());
 
     if (use_color) {
         color_name = "\033[33;1m";
@@ -86,7 +87,7 @@ int main(int argc, char** argv)
     }
 
     Vector<String> trail;
-    print("json", json, trail);
+    print("json", json.value(), trail);
     return 0;
 }
 
@@ -116,7 +117,6 @@ static void print(const String& name, const JsonValue& value, Vector<String>& tr
     }
     switch (value.type()) {
     case JsonValue::Type::Null:
-    case JsonValue::Type::Undefined:
         printf("%s", color_null);
         break;
     case JsonValue::Type::Bool: