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.cpp

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2024-2025, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/StorageAPI/StorageShelf.h>
+
+namespace Web::StorageAPI {
+
+// https://storage.spec.whatwg.org/#create-a-storage-shelf
+StorageShelf::StorageShelf(StorageType type)
+{
+    // 1. Let shelf be a new storage shelf.
+    // 2. Set shelf’s bucket map["default"] to the result of running create a storage bucket with type.
+    bucket_map.set("default"_string, StorageBucket { type });
+    // 3. Return shelf.
+}
+
+}