Should warn on focusable with no role

[ghost]

If an author creates an item on the page that's focusable by adding tabindex=0, and it's not already a form control, it should have a role - and specifically it should have a "widget" role like button or checkbox or one of the "composite control" roles grid, griditem, listbox, or option.

This should probably be an Error and not a warning if we're sure it has a click handler - because JAWS and NVDA will not make an Enter or Space trigger a mouse click if you focus on that item.

If we don't know if it has a click handler, it could be a warning. Probably better to have a role, but if it's not really a widget that has an action, then it won't affect anything.

[alice]

Implementation note: also check for an implicit role.