Consider what to do about <hr> elements inside <select>

See:
- https://developer.chrome.com/en/blog/hr-in-select/
- https://adrianroselli.com/2023/10/splitting-within-selects.html

I'm not sure what the implications of this are for axe-core. From a quick test it looks like `hr` doesn't get rendered in browsers that don't support it. That's probably different if you DOM append nodes with React or the like. Axe has no rules to check for native select elements, but it will look at `role=listbox`. We're going to need to check what the implications of this are.