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

Libraries/LibGUI/JsonArrayModel.cpp

@@ -42,8 +42,9 @@ void JsonArrayModel::update()
 
     auto json = JsonValue::from_string(file->read_all());
 
-    ASSERT(json.is_array());
-    m_array = json.as_array();
+    ASSERT(json.has_value());
+    ASSERT(json.value().is_array());
+    m_array = json.value().as_array();
 
     did_update();
 }