@route directive and source generator for route name constants

[tomasherceg]

Registering all routes individually in DotvvmStartup.cs is painful. I've added an option to use @route directive in DotHTML markup files and have the route auto-generated using source generator.

The results looks as follows:

using DotVVM.Framework.Routing;

namespace DotVVM.Samples.Common
{
    public static class DotvvmRoutes
    {
        // constants for all route names (generated from relative path in the project directory)
        public const string Default = nameof(Default);

        // generated method that will be called from DotvvmStartup to register all routes
        public static void RegisterRoutes(DotvvmRouteTable routes) {
            routes.Add("Default", "test", "Views/Default.dothtml");
        }
    }
}