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/LibCore/EventLoop.cpp

@@ -105,13 +105,13 @@ public:
             auto request = m_socket->read(length);
 
             auto request_json = JsonValue::from_string(request);
-            if (!request_json.is_object()) {
+            if (!request_json.has_value() || !request_json.value().is_object()) {
                 dbg() << "RPC client sent invalid request";
                 shutdown();
                 return;
             }
 
-            handle_request(request_json.as_object());
+            handle_request(request_json.value().as_object());
         };
     }
     virtual ~RPCClient() override