[ic-navigation-group] add prop `selected` to indicate selected child element

[gd2910]

Summary

Add functionality to have the nav-group show a selected state exactly like a selected ic-navigation-item as below

This would allow a developer to indicate which group a user is situated after selecting a child.

[MI6-255]

Add it as a prop and make it know if the children are selected, looking at if we can make it work with routing too, but if not then developers still have the prop. Include a code example of both too