[menu] Consider reusing `<menu>` and `<nav>` HTML elements

Instead of introducing a brand new elements like <menubar> element, we could consider reusing <menu> for the regular, non-navigation menubar example referenced by ARIA's authoring practices: https://www.w3.org/WAI/ARIA/apg/patterns/menubar/examples/menubar-editor/. This would need to be done with some sort of opt-in (much like appearance: base-select does for the existing <select> element, to make it customizable), which could either be in CSS or an HTML attribute. We could also maybe consider reusing <nav> for the navigation bar use case, but this is related to https://github.com/openui/open-ui/issues/1193.

It seems nice to reuse these elements if possible, just because they have nice short names and we'd be adding less new clutter to the platform, but I think there are tradeoffs, and discussing this with WHATWG folks will be important as time passes.

Start with the linked ARIA menubar example and related issue #1193; compare whether

or can cover each use case and review the proposed CSS or HTML opt-in tradeoffs. Done means reaching a clear recommendation about reuse and identifying what discussion with WHATWG would be needed.