LibWeb: Implement HTMLOrSVGElement.tabIndex

Author: https://github.com/Lubrsi
Commit: 1473bc9169
Pull-request: https://github.com/SerenityOS/serenity/pull/15964
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/HTML/HTMLSelectElement.h

@@ -61,6 +61,9 @@ private:
 
     virtual void visit_edges(Cell::Visitor&) override;
 
+    // ^DOM::Element
+    virtual i32 default_tab_index_value() const override;
+
     JS::GCPtr<HTMLOptionsCollection> m_options;
 };