Allow overlays to trap the tab order.

[Westbrook]

As per https://codepen.io/samthor/live/b459e77f8dc7fe8a32925866bf4e0628 we could leverage shadow DOM to implement tab trapping of overlaid content with a relatively low lift. Questions we'd want to have answered:

• Is trapping off of body as is done in the example to invasive?
• When should we trap? For instance, the click-content of an overlay-trigger likely wants for trapping all the time, but the sp-dropdown element does not.
• Should trapping be an explicit part of the Overlay API or implicit to an overlay type?

related #710