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.cpp

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2024, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/Realm.h>
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/HTML/DataTransfer.h>
+
+namespace Web::HTML {
+
+JS_DEFINE_ALLOCATOR(DataTransfer);
+
+JS::NonnullGCPtr<DataTransfer> DataTransfer::construct_impl(JS::Realm& realm)
+{
+    return realm.heap().allocate<DataTransfer>(realm, realm);
+}
+
+DataTransfer::DataTransfer(JS::Realm& realm)
+    : PlatformObject(realm)
+{
+}
+
+DataTransfer::~DataTransfer() = default;
+
+void DataTransfer::initialize(JS::Realm& realm)
+{
+    Base::initialize(realm);
+    set_prototype(&Bindings::ensure_web_prototype<Bindings::DataTransferPrototype>(realm, "DataTransfer"_fly_string));
+}
+
+}

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;
+};
+
+}

Userland/Libraries/LibWeb/HTML/DataTransfer.idl

@@ -0,0 +1,19 @@
+// https://html.spec.whatwg.org/multipage/dnd.html#datatransfer
+[Exposed=Window]
+interface DataTransfer {
+    constructor();
+
+    // FIXME: attribute DOMString dropEffect;
+    // FIXME: attribute DOMString effectAllowed;
+
+    // FIXME: [SameObject] readonly attribute DataTransferItemList items;
+
+    // FIXME: undefined setDragImage(Element image, long x, long y);
+
+    // old interface
+    // FIXME: readonly attribute FrozenArray<DOMString> types;
+    // FIXME: DOMString getData(DOMString format);
+    // FIXME: undefined setData(DOMString format, DOMString data);
+    // FIXME: undefined clearData(optional DOMString format);
+    // FIXME: [SameObject] readonly attribute FileList files;
+};