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.cpp

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/DOM/CustomEvent.h>
+
+namespace Web::DOM {
+
+CustomEvent::CustomEvent(FlyString const& event_name)
+    : Event(event_name)
+{
+}
+
+CustomEvent::~CustomEvent()
+{
+}
+
+void CustomEvent::visit_edges(JS::Cell::Visitor& visitor)
+{
+    visitor.visit(m_detail);
+}
+
+// https://dom.spec.whatwg.org/#dom-customevent-initcustomevent
+void CustomEvent::init_custom_event(String const& type, bool bubbles, bool cancelable, JS::Value detail)
+{
+    // 1. If this’s dispatch flag is set, then return.
+    if (dispatched())
+        return;
+
+    // 2. Initialize this with type, bubbles, and cancelable.
+    initialize(type, bubbles, cancelable);
+
+    // 3. Set this’s detail attribute to detail.
+    m_detail = detail;
+}
+
+}