Introduce new type parameter to navigation item

[zecar]

Is your feature request related to a problem? Please describe

I am displaying a custom navigation for a multi-level course structure and i have no way of knowing if a route is to a folder or to a .md file

Describe the solution you'd like

ideally, the navigation item would have a type property with values like article | file or folder | directory in order to be able to customize the navigation

Describe alternatives you've considered

right now i'm using the logic of "if route has no children, it must be an article" but that is not correct because it could be an empty folder