LibWeb: Add an empty DataTransfer IDL implementation

This does not implement any of the IDL methods, but GitHub requires the
interface exists to upload files via an <input type="file"> element.
Their JS handles uploads via this element and via drag-and-drop in one
function, and check if the uploaded file is `instanceof DataTransfer` to
decide how to handle it.

Userland/Libraries/LibWeb/HTML/DataTransfer.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2024, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Forward.h>
+#include <LibWeb/Bindings/PlatformObject.h>
+
+namespace Web::HTML {
+
+class DataTransfer : public Bindings::PlatformObject {
+    WEB_PLATFORM_OBJECT(DataTransfer, Bindings::PlatformObject);
+    JS_DECLARE_ALLOCATOR(DataTransfer);
+
+public:
+    static JS::NonnullGCPtr<DataTransfer> construct_impl(JS::Realm&);
+    virtual ~DataTransfer() override;
+
+private:
+    DataTransfer(JS::Realm&);
+
+    virtual void initialize(JS::Realm&) override;
+};
+
+}