Support controlling ActionMenu position (anchor side)

[iansan5653]

ActionMenu always renders below its anchor element, and this cannot be overridden. This would be very useful to control, particularly to be able to position submenus to the right of the anchor:

In the above screenshot, we'd like to render the menu to the right by default, as recommended by the Primer design guidelines.

ActionMenu.Overlay does forward unused props down to the underlying Overlay component, and Overlay does accept an anchorSide prop. This would appear to allow custom positioning, but this prop only controls the transition direction on open -- not the position.

A simple solution would be to add a side prop and forward it to the AnchoredOverlay that powers the menu.