feat(ThreadChannel): add fetchStarterMessage (#6488)

Co-authored-by: Vlad Frangu <kingdgrizzle@gmail.com>

src/structures/ThreadChannel.js

@@ -253,6 +253,17 @@ class ThreadChannel extends Channel {
     return members.get(this.ownerId) ?? null;
   }
 
+  /**
+   * 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>
+   * @param {BaseFetchOptions} [options] Additional options for this fetch
+   * @returns {Promise<Message>}
+   */
+  fetchStarterMessage(options) {
+    return this.channel.messages.fetch(this.id, options);
+  }
+
   /**
    * The options used to edit a thread channel
    * @typedef {Object} ThreadEditData