Support an alternative selection mode for Stack- and Grid-based tables

[reedes]

The current selection mode is a simple tap to select or unselect. No EditMode required on iOS. It doesn't appear to interfere with context or swipe menus.

But ideally there would be an alternative selection mode that matches the native selection of List tables.

On macOS, that involves discarding the selection if the user clicks in a non-row portion of the control. Selecting multiple items requires the Cmd key. Multiple rows can be selected via click and drag.

On iOS, the native selection on List tables typically requires entering EditMode. That might be avoided altogether in the Stack- and Grid-based tables.