backport(TextBasedChannel): add lastPinTimestamp and lastPinAt (#2870)

And clarify Client#channelPinsUpdate's 'time' parameter.

src/structures/TextChannel.js

@@ -37,6 +37,12 @@ class TextChannel extends GuildChannel {
      */
     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;
+
     /**
      * The ratelimit per user for this channel
      * @type {number}
@@ -115,6 +121,7 @@ class TextChannel extends GuildChannel {
 
   // These are here only for documentation purposes - they are implemented by TextBasedChannel
   /* eslint-disable no-empty-function */
+  get lastPinAt() {}
   send() { }
   sendMessage() { }
   sendEmbed() { }