added a configurable default mode so we don't have to start in INSERT

[MatrixManAtYrService]

If accepted, this would fix #1631

Usage looks like this:

    app = Application(
        key_bindings=kb, layout=Layout(root_container), editing_mode=EditingMode.VI
    )
    app.vi_state.default_input_mode = InputMode.NAVIGATION
    app.reset()
    app.run()

I'm not confident about the bigger picture here. Like maybe there's a way to do this that fits better with the many other ways to use prompt-toolkit. All I can say is that it works well in my case.

[MatrixManAtYrService]

That failing test... I don't think it has anything to do with my change.