restrict to certain vim modes

[MatrixManAtYrService]

I am working on an app that uses prompt-toolkit to let users delete subsets of a string, but it's invalid to add anything to that string. In this case I want the prompt to be stuck in NAVIGATION mode.

After making this change to prompt toolkit, I can use it like so:

app.vi_state.allowed_input_modes = [InputMode.NAVIGATION]
app.run()

If there's a better way to achieve this, I'd be interested to hear about it. Thanks.