LibWeb: Add ol start and li value attributes support

Author: https://github.com/bplaat
Commit: 1cdbfc2ff1
Pull-request: https://github.com/SerenityOS/serenity/pull/21408
Reviewed-by: https://github.com/AtkinsSJ

Userland/Libraries/LibWeb/Layout/TreeBuilder.h

@@ -25,6 +25,8 @@ private:
         bool has_svg_root = false;
     };
 
+    i32 calculate_list_item_index(DOM::Node&);
+
     ErrorOr<void> create_layout_tree(DOM::Node&, Context&);
 
     void push_parent(Layout::NodeWithStyle& node) { m_ancestor_stack.append(node); }