feat: Allow OnCustomAction on NavMenuGroup

[mrpmorris]

Scenario: NavMenu is collapsed, user selects a group

The correct default behaviour is for the nav menu to expand and the group to appear expanded. My requirement is that a popup menu should appear instead.

Proposal:
Add a callback to FluentNavMenuGroup EventCallback<FluentNavMenuGroupSelectingArgs> OnSelecting

Ordinarily, the group is not selectable if it doesn't have a value set in its Href parameter. This would be changed so that it is selectable if Href is set, or OnSelecting is set.

The args classes would be

public class ....EventArgs : EventArgs
{
  public FluentNavMenuGroup Group { get; }
  public bool Cancelled { get; }
  public void Cancel()
  {
    Cancelled = true;
  }
}

where Cancelled would default to true if the group had no Href.

This would allow the user to add an OnSelecting action to the group, handle it with a custom action, and then cancel the original navigation if appropriate.

If executing OnCustomAction calls Cancel then the item will not be selected