LibWeb: Add MediaList

This is a list of MediaQuery objects. Not to be confused with
`MediaQueryList`, which is concerned with firing events when a media
query's match-state changes.

Userland/Libraries/LibWeb/CSS/MediaList.cpp

@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2021, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/CSS/MediaList.h>
+#include <LibWeb/CSS/Parser/Parser.h>
+
+namespace Web::CSS {
+
+MediaList::MediaList(NonnullRefPtrVector<MediaQuery>&& media)
+    : m_media(move(media))
+{
+}
+
+MediaList::~MediaList()
+{
+}
+
+// https://www.w3.org/TR/cssom-1/#dom-medialist-mediatext
+String MediaList::media_text() const
+{
+    StringBuilder builder;
+    builder.join(", ", m_media);
+    return builder.to_string();
+}
+
+// https://www.w3.org/TR/cssom-1/#dom-medialist-mediatext
+void MediaList::set_media_text(String const& text)
+{
+    m_media.clear();
+    if (text.is_empty())
+        return;
+    m_media = parse_media_query_list({}, text);
+}
+
+// https://www.w3.org/TR/cssom-1/#dom-medialist-item
+Optional<String> MediaList::item(size_t index) const
+{
+    if (index >= length())
+        return {};
+
+    return m_media[index].to_string();
+}
+
+// https://www.w3.org/TR/cssom-1/#dom-medialist-appendmedium
+void MediaList::append_medium(String medium)
+{
+    auto m = parse_media_query({}, medium);
+    if (!m)
+        return;
+    if (m_media.contains_slow(*m))
+        return;
+    m_media.append(m.release_nonnull());
+}
+
+// https://www.w3.org/TR/cssom-1/#dom-medialist-deletemedium
+void MediaList::delete_medium(String medium)
+{
+    auto m = parse_media_query({}, medium);
+    if (!m)
+        return;
+    m_media.remove_all_matching([&](auto& existing) -> bool {
+        return m->to_string() == existing->to_string();
+    });
+    // FIXME: If nothing was removed, then throw a NotFoundError exception.
+}
+
+}