LibWeb: Fire MediaQueryListEvents when an MQL's match-state changes

The HTML event loop does a check for MQL match-state changes and
dispatches the events. This requires us to keep a list of MQLs on the
Document.

Userland/Libraries/LibWeb/CSS/MediaQueryListEvent.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/DOM/Event.h>
+
+namespace Web::CSS {
+
+struct MediaQueryListEventInit : public DOM::EventInit {
+    String media { "" };
+    bool matches { false };
+};
+
+class MediaQueryListEvent : public DOM::Event {
+public:
+    using WrapperType = Bindings::MediaQueryListEventWrapper;
+
+    static NonnullRefPtr<MediaQueryListEvent> create(FlyString const& event_name, MediaQueryListEventInit const& event_init = {})
+    {
+        return adopt_ref(*new MediaQueryListEvent(event_name, event_init));
+    }
+    static NonnullRefPtr<MediaQueryListEvent> create_with_global_object(Bindings::WindowObject&, FlyString const& event_name, MediaQueryListEventInit const& event_init)
+    {
+        return MediaQueryListEvent::create(event_name, event_init);
+    }
+
+    virtual ~MediaQueryListEvent() override = default;
+
+    String const& media() const { return m_media; }
+    bool matches() const { return m_matches; }
+
+protected:
+    MediaQueryListEvent(FlyString const& event_name, MediaQueryListEventInit const& event_init)
+        : DOM::Event(event_name, event_init)
+        , m_media(event_init.media)
+        , m_matches(event_init.matches)
+    {
+    }
+
+    String m_media;
+    bool m_matches;
+};
+}

Userland/Libraries/LibWeb/CSS/MediaQueryListEvent.idl

@@ -0,0 +1,13 @@
+#import <DOM/Event.idl>
+
+interface MediaQueryListEvent : Event {
+    constructor(CSSOMString type, optional MediaQueryListEventInit eventInitDict = {});
+
+    readonly attribute CSSOMString media;
+    readonly attribute boolean matches;
+};
+
+dictionary MediaQueryListEventInit : EventInit {
+    CSSOMString media = "";
+    boolean matches = false;
+};