feat(Message): add call property

Ref: discord/discord-api-docs@f7393bd4

lib/structures/Message.js

@@ -528,6 +528,17 @@ class Message extends Base {
              */
             this.poll = data.poll;
         }
+
+        if(data.call !== undefined) {
+            /**
+             * The call associated with this message
+             * @type {Message.Call?}
+             */
+            this.call = {
+                participants: data.call.participants,
+                endedTimestamp: data.call.ended_timestamp !== null ? Date.parse(data.call.ended_timestamp) : null
+            };
+        }
     }
 
     /**
@@ -801,6 +812,7 @@ class Message extends Base {
             "application",
             "attachments",
             "author",
+            "call",
             "content",
             "editedTimestamp",
             "embeds",
@@ -849,3 +861,9 @@ module.exports = Message;
  * @typedef Message.MessageSnapshot
  * @prop {Message} message The message snapshot. Note that this object contains the minimal subset of message data.
  */
+/**
+ * An object describing a call in a message
+ * @typedef Message.Call
+ * @prop {Array<String>} participants An array of user IDs that took part in the call
+ * @prop {Number?} endedTimestamp The time when the call ended
+ */