fix: Make sure to update aria-expanded on menu

Part-of: <!88>

theme/static/js/navbar-burger.js

@@ -19,6 +19,12 @@ document.addEventListener('DOMContentLoaded', () => {
         el.classList.toggle('is-active');
         $target.classList.toggle('is-active');
 
+        el.setAttribute(
+          'aria-expanded', 
+          el.getAttribute('aria-expanded') === 'true' 
+          ? 'false' 
+          : 'true'
+        );
       });
     });
   }