Easily use custom fragment for incremental list

I'm not sure if this is a good idea, but what about adding a variable in the metadata, such as custom-fragments=True, so that all <li class="fragment"> fragments generated by bullet lists become class="fragment custom"?

Currently, I'm using the following JavaScript code when compiling the document:

document.addEventListener("DOMContentLoaded", function () {
  document.querySelectorAll('.fragment').forEach(function (element) {
    element.classList.add('custom');
    
    // Avoid turning elements inside `agenda` https://github.com/andrie/reveal-auto-agenda into `.fragment .custom`.
    if (element.closest('.agenda')) {
      element.className = "agenda list";
    }
  });
});

Originally posted by @ChoCho66 in https://github.com/quarto-dev/quarto-cli/discussions/9643#discussioncomment-10910840