LibWeb: Add initial support for CustomEvent

This is used surprisingly often. For example, it is used by a core
YouTube library called Structured Page Fragments.

It allows you to manually dispatch an event with arbitrary data
attached to it.

The only thing missing from this implementation is the constructor.
This is because WrapperGenerator is currently missing dictionary
capabilities.

Userland/Libraries/LibWeb/DOM/CustomEvent.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/DOM/Event.h>
+
+namespace Web::DOM {
+
+// https://dom.spec.whatwg.org/#customevent
+class CustomEvent : public Event {
+public:
+    using WrapperType = Bindings::CustomEventWrapper;
+
+    static NonnullRefPtr<CustomEvent> create(FlyString const& event_name)
+    {
+        return adopt_ref(*new CustomEvent(event_name));
+    }
+
+    virtual ~CustomEvent() override;
+
+    // https://dom.spec.whatwg.org/#dom-customevent-detail
+    JS::Value detail() const { return m_detail; }
+
+    void visit_edges(JS::Cell::Visitor&);
+
+    void init_custom_event(String const& type, bool bubbles, bool cancelable, JS::Value detail);
+
+private:
+    CustomEvent(FlyString const&);
+
+    // https://dom.spec.whatwg.org/#dom-customevent-initcustomevent-type-bubbles-cancelable-detail-detail
+    JS::Value m_detail { JS::js_null() };
+};
+
+}