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/HTMLAreaElement.h

@@ -25,6 +25,7 @@ private:
 
     // ^DOM::Element
     virtual void parse_attribute(FlyString const& name, String const& value) override;
+    virtual i32 default_tab_index_value() const override;
 
     // ^HTML::HTMLHyperlinkElementUtils
     virtual DOM::Document& hyperlink_element_utils_document() override { return document(); }