feat(Message): add crosspostable property (#4903)

Co-authored-by: Advaith <advaithj1@gmail.com>
Co-authored-by: Alex Hîncu <teesealz@gmail.com>

src/structures/Message.js

@@ -425,6 +425,23 @@ class Message extends Base {
     );
   }
 
+  /**
+   * Whether the message is crosspostable by the client user
+   * @type {boolean}
+   * @readonly
+   */
+  get crosspostable() {
+    return (
+      this.channel.type === 'news' &&
+      !this.flags.has(MessageFlags.FLAGS.CROSSPOSTED) &&
+      this.type === 'DEFAULT' &&
+      this.channel.viewable &&
+      this.channel.permissionsFor(this.client.user).has(Permissions.FLAGS.SEND_MESSAGES) &&
+      (this.author.id === this.client.user.id ||
+        this.channel.permissionsFor(this.client.user).has(Permissions.FLAGS.MANAGE_MESSAGES))
+    );
+  }
+
   /**
    * Options that can be passed into editMessage.
    * @typedef {Object} MessageEditOptions