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

Meta/Lagom/TestJson.cpp

@@ -31,7 +31,8 @@
 int main(int, char**)
 {
     auto value = JsonValue::from_string("{\"property\": \"value\"}");
-    printf("parsed: _%s_\n", value.to_string().characters());
-    printf("object.property = '%s'\n", value.as_object().get("property").to_string().characters());
+    ASSERT(value.has_value());
+    printf("parsed: _%s_\n", value.value().to_string().characters());
+    printf("object.property = '%s'\n", value.value().as_object().get("property").to_string().characters());
     return 0;
 }