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

## 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.

Start by locating the ic-navigation-group component and its existing selected-state behavior, then compare it with the selected ic-navigation-item behavior described in the issue. Done means the navigation group accepts a selected prop and displays the same selected state when that prop is set.

Written by the indexing model from the issue text.