Add message/channel/guild acknowledging (#1239)

* add acking

* 👀

* Update RESTMethods.js

* Update TextBasedChannel.js

* Update RESTMethods.js

* Update Guild.js

* Update TextBasedChannel.js

* Update Message.js

* super shitty names

* Update GroupDMChannel.js

* Update DMChannel.js

* Update TextChannel.js

src/structures/Guild.js

@@ -770,6 +770,15 @@ class Guild {
     return this.client.rest.methods.deleteGuild(this);
   }
 
+  /**
+   * Marks all messages in this guild as read
+   * <warn>This is only available when using a user account.</warn>
+   * @returns {Promise<Guild>} this guild
+   */
+  acknowledge() {
+    return this.client.rest.methods.ackGuild(this);
+  }
+
   /**
    * Whether this Guild equals another Guild. It compares all properties, so for most operations
    * it is advisable to just compare `guild.id === guild2.id` as it is much faster and is often