Add loading state on nested router

[pedrobslisboa]

Description

This PR adds a simple loading to the RouteDefinition

How

For example:

    {
      path: "/:id",
      loading: Some(() => <StudentsLoading />),
      page:
        Some(
          (~dynamicParams, ~queryParams as _) => {
            let id =
              Router.DynamicParams.find("id", dynamicParams)
              |> Option.map(int_of_string);
            <StudentsPage id />;
          },
        ),
    }