Review accessibility of our listless menus

Following the lovely design's static layout perhaps a bit too closely, i implemented it as shown with <div>s rather than <ul>s and unadorned <a> instead of <li> tags. This didn't concern me at all from an accessibility perspective until getting to the sub-menus— this, it seems, should have a semantic way of being 'below' their parent items in the hierarchy.

My implementation: https://gitlab.com/agaric/patternlibrary/blob/master/agaric/templates/navigation/menu.html.twig

Original: https://cgit.drupalcode.org/bulma/tree/templates/navigation/menu.html.twig