LibWeb: Add Storage interface and window.localStorage

This is a naive-but-somewhat-functional initial implementation of
HTML Storage.

Note that there is no persistence yet, everything is in-process only,
and one local Storage object per origin.

Userland/Libraries/LibWeb/HTML/Storage.h

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/HashMap.h>
+#include <AK/RefCounted.h>
+#include <LibWeb/Bindings/Wrappable.h>
+#include <LibWeb/DOM/ExceptionOr.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::HTML {
+
+class Storage
+    : public RefCounted<Storage>
+    , public Bindings::Wrappable {
+public:
+    using WrapperType = Bindings::StorageWrapper;
+
+    static NonnullRefPtr<Storage> create();
+    ~Storage();
+
+    size_t length() const;
+    String key(size_t index);
+    String get_item(String const& key) const;
+    DOM::ExceptionOr<void> set_item(String const& key, String const& value);
+    void remove_item(String const& key);
+    void clear();
+
+    Vector<String> supported_property_names() const;
+
+private:
+    Storage();
+
+    void reorder();
+    void broadcast(String const& key, String const& old_value, String const& new_value);
+
+    OrderedHashMap<String, String> m_map;
+};
+
+}
+
+namespace Web::Bindings {
+
+StorageWrapper* wrap(JS::GlobalObject&, HTML::Storage&);
+
+}