Starting mathicsscript in vi editing mode

[mickmcq]

Is your feature request related to a problem? Please describe.
mathicsscript starts in Emacs mode if I start it with mathicsscript or mathicsscript --readline Prompt. If I start it with mathicsscript --readline GNU it reads my ~/.inputrc file and hence respects my preferred edit mode. But there is no such configuration file for prompt_toolkit as far as I can tell. I would need to insert code into mathicsscript to enable the switch.

Describe the solution you'd like
I would like a command-line option to either (A) enable Vi-mode at startup, or (B) enable a fragment of python code to be inserted after startup but before interactivity becomes available. The fragment I would want to run would be something like app.editing_mode = EditingMode.VI but it depends on the termshell_prompt.py file having already imported EditingMode from prompt_toolkit.enums and, of course, prompt_toolkit remaining the default.

Describe alternatives you've considered
I have included the code mentioned above in termshell_prompt.py after line 162. That's fine until mathicsscript is updated! Then I have to add it again, possibly in a different place. Another alternative is to use GNU readline, which would respect my .inputrc file, but that provides a much worse user experience in terms of syntax highlighting and autocompletion than does prompt_toolkit.

Additional context
I'm afraid option (A) may become obsolete if prompt_toolkit gains a configuration file, but option (B) might be more long-lived, although it could also be abused.

I searched for such a feature in the documentation and it may be there since, as a newb, I don't fully understand the documentation. It seems that the options for adding files and expressions all require valid mathics expressions, not raw Python code. I see that there is a "Python extensions" directory but can find no documentation about it.