refactor(Webhook): make token not enumerable (#2566)

* Hide Webhook#token (consistency with Client#token)

* Make `Webhook#token` writable

* fix: devsnek's requested change

Webhook#token must be both writable and configurable.

src/structures/Webhook.js

@@ -28,7 +28,7 @@ class Webhook {
      * The token for the webhook
      * @type {string}
      */
-    this.token = data.token;
+    Object.defineProperty(this, 'token', { value: data.token, writable: true, configurable: true });
 
     /**
      * The avatar for the webhook