[menu] [openable] allow author choice of opening a submenu inline or as a popup

We are likely all familiar with the use case for submenu popups being invoked on hover/focus or click of a menu's menuitem.

I've also seen use cases where authors have created "submenus" which are more like expand/collapse groups within a menu list.

I mention this because it could be another use case for the proposed openable attribute, and could allow for something like:

<menulist>
  ...
  <menuitem command=toggle commandfor=g>whatever</menuitem>
  <fieldset openable id=g>
    <menuitem>A</menuitem>
    <menuitem>B</menuitem>
    <menuitem>C</menuitem>
  </fieldset>
  ...
</menulist>

this could allow for an expand/collapse state on the menuitem with the command=toggle. it would not invoke it as a popover. ideally, this could allow for an automatic naming mechanism for the fieldset (allowing it to be exposed as a role=group with accName "whatever"). this automatic naming helping to make sure that the group start/end points were defined - and if someone wanted to give the fieldset a more specific name when the set was in the expanded state, they could use the legend element to do so. (or whatever the elements end up being for grouping/naming said group for menuitems)

cc @lukewarlow

Start by reviewing the proposed openable attribute and the menu, menuitem, fieldset, legend, and command=toggle examples in this issue. Determine whether inline expansion and popup behavior can share this model, including automatic group naming and accessibility semantics. Done means reaching a concrete specification decision for the behavior and naming model.