LibWeb: Correctly initialize Storage objects on the Document

Instead of storing all storage objects in static memory, we now
follow the the spec by lazily creating a unique Storage object
on each document object.

Each Storage object now holds a 'proxy' to the underlying backing
storage. For now, this proxy is simply a reference to the backing
object. In the future, it will need to be some type of interface
object that stores on a SQLite database or similar.

Session storage is now correctly stored / tracked as part of the
TraversableNavigable object.

Local storage is still stored in a static map, but eventually this
should be factored into something that is stored at the user agent
level.

Libraries/LibWeb/StorageAPI/StorageShelf.h

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2024-2025, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/HashMap.h>
+#include <AK/String.h>
+#include <LibWeb/Forward.h>
+#include <LibWeb/StorageAPI/StorageBottle.h>
+#include <LibWeb/StorageAPI/StorageType.h>
+
+namespace Web::StorageAPI {
+
+// https://storage.spec.whatwg.org/#storage-shelf
+// A storage shelf exists for each storage key within a storage shed. It holds a bucket map, which is a map of strings to storage buckets.
+using BucketMap = OrderedHashMap<String, StorageBucket>;
+
+struct StorageShelf {
+    explicit StorageShelf(StorageType);
+
+    BucketMap bucket_map;
+};
+
+}