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/HTMLInputElement.h

@@ -123,6 +123,9 @@ private:
     virtual void legacy_cancelled_activation_behavior() override;
     virtual void legacy_cancelled_activation_behavior_was_not_called() override;
 
+    // ^DOM::Element
+    virtual i32 default_tab_index_value() const override;
+
     virtual void visit_edges(Cell::Visitor&) override;
 
     static TypeAttributeState parse_type_attribute(StringView);