Add default BlazorBindings App class

[Dreamescaper]

When used with BlazorBindings, in most cases App class looks like this:

    public class App : Application
    {
        public App(MauiBlazorBindingsRenderer renderer)
        {
            renderer.AddComponent<AppShell>(this);
        }
    }

It is not advised to interact with MauiBlazorBindingsRenderer in user's code, therefore we should add BlazorBindingsApp<T> class, which will render T component as a root.