backport: WEBHOOKS_UPDATE event (#2779)

src/client/websocket/packets/handlers/WebhooksUpdate.js

@@ -0,0 +1,19 @@
+const AbstractHandler = require('./AbstractHandler');
+const { Events } = require('../../../../util/Constants');
+
+class WebhooksUpdate extends AbstractHandler {
+  handle(packet) {
+    const client = this.packetManager.client;
+    const data = packet.d;
+    const channel = client.channels.get(data.channel_id);
+    if (channel) client.emit(Events.WEBHOOKS_UPDATE, channel);
+  }
+}
+
+/**
+ * Emitted whenever a guild text channel has its webhooks changed.
+ * @event Client#webhookUpdate
+ * @param {TextChannel} channel The channel that had a webhook update
+ */
+
+module.exports = WebhooksUpdate;