feat: Guild Forum channel

https://github.com/discordjs/discord.js/pull/8651

src/structures/PartialGroupDMChannel.js

@@ -17,6 +17,10 @@ const DataResolver = require('../util/DataResolver');
 class PartialGroupDMChannel extends Channel {
   constructor(client, data) {
     super(client, data);
+
+    // No flags are present when fetching partial group DM channels.
+    this.flags = null;
+
     /**
      * The name of this Group DM Channel
      * @type {?string}