Improve/fix navigation to the Lesson Feature

As the title says, the navigation to the _lessonFeature_ needs some revising. Currently it works, but it is not possible to add a second destination to the "presumed nav graph". _lessonFeature_ contains a _navigation composable_ having the _lesson id_ as argument and the _Lesson start screen_ as the content. But the _navigation composable_ should be instead a _nested nav graph_ that has as "children" _composables_ with all the screens related to a lesson (start, progress, finish screens). Also, currently, passing the lesson id argument to the specific _navigation route_ involves hard coding the route, which isn't type safe. I would have fixed the first issue myself back then, but I couldn't find a correct way of passing arguments to a nested nav graph.