LibWeb: Add support for document.lastModified

Author: https://github.com/shannonbooth
Commit: ce341c0230
Pull-request: https://github.com/SerenityOS/serenity/pull/23799

Userland/Libraries/LibWeb/DOM/Document.h

@@ -1,7 +1,7 @@
 /*
  * Copyright (c) 2018-2023, Andreas Kling <kling@serenityos.org>
  * Copyright (c) 2021-2023, Linus Groh <linusg@serenityos.org>
- * Copyright (c) 2023, Shannon Booth <shannon@serenityos.org>
+ * Copyright (c) 2023-2024, Shannon Booth <shannon@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -14,6 +14,7 @@
 #include <AK/String.h>
 #include <AK/Vector.h>
 #include <AK/WeakPtr.h>
+#include <LibCore/DateTime.h>
 #include <LibCore/Forward.h>
 #include <LibJS/Forward.h>
 #include <LibURL/URL.h>
@@ -323,6 +324,8 @@ public:
     HTML::DocumentReadyState readiness() const { return m_readiness; }
     void update_readiness(HTML::DocumentReadyState);
 
+    String last_modified() const;
+
     [[nodiscard]] JS::GCPtr<HTML::Window> window() const { return m_window; }
 
     void set_window(HTML::Window&);
@@ -872,6 +875,8 @@ private:
 
     Vector<JS::NonnullGCPtr<DOM::ShadowRoot>> m_shadow_roots;
 
+    Optional<Core::DateTime> m_last_modified;
+
     u64 m_dom_tree_version { 0 };
 
     // https://drafts.csswg.org/css-position-4/#document-top-layer