LibWeb: Stub CacheStorage interface

With this change we can again open login form https://discord.com/login
instead of failing in js because `caches.open()` is not defined.

Libraries/LibWeb/ServiceWorker/CacheStorage.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2025, Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/Bindings/PlatformObject.h>
+#include <LibWeb/WebIDL/Promise.h>
+
+namespace Web::ServiceWorker {
+
+// https://w3c.github.io/ServiceWorker/#cachestorage-interface
+class CacheStorage : public Bindings::PlatformObject {
+    WEB_PLATFORM_OBJECT(CacheStorage, Bindings::PlatformObject);
+    GC_DECLARE_ALLOCATOR(CacheStorage);
+
+public:
+    GC::Ref<WebIDL::Promise> open(String const& cache_name);
+
+private:
+    explicit CacheStorage(JS::Realm&);
+
+    virtual void initialize(JS::Realm&) override;
+};
+
+}