LibWeb: Derive implicit aria-level for h1–h6 from tag name

Author: https://github.com/Dowsley 🔰
Commit: c6b289c3dc
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/6256
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/Internals/Internals.idl

@@ -49,6 +49,7 @@ interface Internals {
 
     DOMString getComputedRole(Element element);
     DOMString getComputedLabel(Element element);
+    DOMString getComputedAriaLevel(Element element);
     unsigned short getEchoServerPort();
 
     undefined setBrowserZoom(double factor);