fix(Invite): fix valueOf returning undefined (#3582)

* Update Invite.js

* Fix typings

* Fix ESLint errors

src/structures/Invite.js

@@ -183,6 +183,10 @@ class Invite extends Base {
       guild: 'guildID',
     });
   }
+
+  valueOf() {
+    return this.code;
+  }
 }
 
 module.exports = Invite;