LibWeb: Move StructuredSerializeOptions to its own header

This largely reduces the number of files needed to be compiled when we
change the MessagePort header.

Userland/Libraries/LibWeb/HTML/StructuredSerializeOptions.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2024, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Vector.h>
+#include <LibJS/Runtime/Object.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/web-messaging.html#structuredserializeoptions
+struct StructuredSerializeOptions {
+    Vector<JS::Handle<JS::Object>> transfer;
+};
+
+}