feat: Guild Forum channel

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

src/structures/ThreadChannel.js

@@ -158,9 +158,8 @@ class ThreadChannel extends Channel {
 
     if ('message_count' in data) {
       /**
-       * The approximate count of messages in this thread
-       * <info>This stops counting at 50. If you need an approximate value higher than that, use
-       * `ThreadChannel#messages.cache.size`</info>
+       * <info>Threads created before July 1, 2022 may have an inaccurate count.
+       * If you need an approximate value higher than that, use `ThreadChannel#messages.cache.size`</info>
        * @type {?number}
        */
       this.messageCount = data.message_count;
@@ -180,6 +179,27 @@ class ThreadChannel extends Channel {
       this.memberCount ??= null;
     }
 
+    if ('total_message_sent' in data) {
+      /**
+       * The number of messages ever sent in a thread, similar to {@link ThreadChannel#messageCount} except it
+       * will not decrement whenever a message is deleted
+       * @type {?number}
+       */
+      this.totalMessageSent = data.total_message_sent;
+    } else {
+      this.totalMessageSent ??= null;
+    }
+
+    if ('applied_tags' in data) {
+      /**
+       * The tags applied to this thread
+       * @type {Snowflake[]}
+       */
+      this.appliedTags = data.applied_tags;
+    } else {
+      this.appliedTags ??= [];
+    }
+
     if (data.member && this.client.user) this.members._add({ user_id: this.client.user.id, ...data.member });
     if (data.messages) for (const message of data.messages) this.messages._add(message);
   }
@@ -225,7 +245,7 @@ class ThreadChannel extends Channel {
 
   /**
    * The parent channel of this thread
-   * @type {?(NewsChannel|TextChannel)}
+   * @type {?(NewsChannel|TextChannel|ForumChannel)}
    * @readonly
    */
   get parent() {
@@ -280,14 +300,16 @@ class ThreadChannel extends Channel {
 
   /**
    * Fetches the message that started this thread, if any.
-   * <info>This only works when the thread started from a message in the parent channel, otherwise the promise will
-   * reject. If you just need the id of that message, use {@link ThreadChannel#id} instead.</info>
+   * <info>The `Promise` will reject if the original message in a forum post is deleted
+   * or when the original message in the parent channel is deleted.
+   * If you just need the id of that message, use {@link ThreadChannel#id} instead.</info>
    * @param {BaseFetchOptions} [options] Additional options for this fetch
    * @returns {Promise<Message|null>}
    */
   // eslint-disable-next-line require-await
   async fetchStarterMessage(options) {
-    return this.parent?.messages.fetch(this.id, options) ?? null;
+    const channel = this.parent?.type === 'GUILD_FORUM' ? this : this.parent;
+    return channel?.messages.fetch({ message: this.id, ...options }) ?? null;
   }
 
   /**
@@ -326,6 +348,7 @@ class ThreadChannel extends Channel {
         rate_limit_per_user: data.rateLimitPerUser,
         locked: data.locked,
         invitable: this.type === 'GUILD_PRIVATE_THREAD' ? data.invitable : undefined,
+        applied_tags: data.appliedTags,
       },
       reason,
     });
@@ -333,6 +356,16 @@ class ThreadChannel extends Channel {
     return this.client.actions.ChannelUpdate.handle(newData).updated;
   }
 
+  /**
+   * Set the applied tags for this channel (only applicable to forum threads)
+   * @param {Snowflake[]} appliedTags The tags to set for this channel
+   * @param {string} [reason] Reason for changing the thread's applied tags
+   * @returns {Promise<GuildForumThreadChannel>}
+   */
+  setAppliedTags(appliedTags, reason) {
+    return this.edit({ appliedTags, reason });
+  }
+
   /**
    * Sets whether the thread is archived.
    * @param {boolean} [archived=true] Whether the thread is archived