LibWeb: Set script force async to false when async changes

Author: https://github.com/jamierocks
Commit: https://github.com/LadybirdBrowser/ladybird/commit/bedb626c299
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/961
Reviewed-by: https://github.com/tcl3 ✅

Tests/LibWeb/Text/input/HTML/script-async-attribute.html

@@ -7,5 +7,9 @@
 
         const script = document.createElement("script");
         println(`script.async = ${script.async}`);
+
+        script.setAttribute("async", "true");
+        script.removeAttribute("async");
+        println(`script.async = ${script.async}`);
     });
 </script>