feat(TextBasedChannel): add lastPinTimestamp and lastPinAt (#2813)

* add lastPinTimestamp

* typings

* use or instead of ternary

src/structures/GroupDMChannel.js

@@ -103,6 +103,12 @@ class GroupDMChannel extends Channel {
      * @type {?Snowflake}
      */
     this.lastMessageID = data.last_message_id;
+
+    /**
+     * The timestamp when the last pinned message was pinned, if there was one
+     * @type {?number}
+     */
+    this.lastPinTimestamp = data.last_pin_timestamp ? new Date(data.last_pin_timestamp).getTime() : null;
   }
 
   /**
@@ -219,6 +225,7 @@ class GroupDMChannel extends Channel {
   // These are here only for documentation purposes - they are implemented by TextBasedChannel
   /* eslint-disable no-empty-function */
   get lastMessage() {}
+  get lastPinAt() {}
   send() {}
   search() {}
   startTyping() {}