docs: Add TypeDef for MessageActivity (#2321)

src/structures/Message.js

@@ -138,7 +138,7 @@ class Message extends Base {
 
     /**
      * Group activity
-     * @type {?Object}
+     * @type {?MessageActivity}
      */
     this.activity = data.activity ? {
       partyID: data.activity.party_id,

src/structures/Presence.js

@@ -1,5 +1,12 @@
 const { ActivityTypes, ActivityFlags } = require('../util/Constants');
 
+/**
+ * Activity sent in a message.
+ * @typedef {Object} MessageActivity
+ * @property {string} [partyID] Id of the party represented in activity
+ * @property {number} [type] Type of activity sent
+ */
+
 /**
  * Represents a user's presence.
  */