feat: add new MessageFlags.FLAGS & User#system (#3603)

* feat: add new FLAGS

* feat: add system property

* typings: add User#system & new MessageFlagsStrings

src/util/MessageFlags.js

@@ -13,6 +13,8 @@ class MessageFlags extends BitField {}
  * * `CROSSPOSTED`
  * * `IS_CROSSPOST`
  * * `SUPPRESS_EMBEDS`
+ * * `SOURCE_MESSAGE_DELETED`
+ * * `URGENT`
  * @type {Object}
  * @see {@link https://discordapp.com/developers/docs/resources/channel#message-object-message-flags}
  */
@@ -20,6 +22,8 @@ MessageFlags.FLAGS = {
   CROSSPOSTED: 1 << 0,
   IS_CROSSPOST: 1 << 1,
   SUPPRESS_EMBEDS: 1 << 2,
+  SOURCE_MESSAGE_DELETED: 1 << 3,
+  URGENT: 1 << 4,
 };
 
 module.exports = MessageFlags;