Support cmd2 in xonsh shell

[anki-code]

Hey!
What do you think about implementing cmd2 support for xonsh shell?

Xonsh supports python-toolkit (default) and readline:

python -m pip install 'xonsh[full]'
xonsh # prompt-toolkit by default
xonsh --shell-type readline  # try readline (less features)

Because of xonsh has modular structure cmd2 support can be implemented as base shell or as a xontrib (plugin).

Personally I'm using python-toolkit very long time and feel the lack of fresh energy and implementing modern features that cmd2 seems to have. I think xonsh is the place where prompt engine can fully unlock their potential. So I'm requesting xonsh shell support here.

[github-actions]

🤖 Hi @anki-code, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.

[github-actions]

🤖 I'm sorry @anki-code, but I was unable to process your request. Please see the logs for more details.

[kmvanbrunt]

Originally our plan was to switch from readline to prompt_toolkit for cmd2 3.0.

You can read why here:
#1351 (comment)

However, the switch to enable Rich support was larger than expected so the prompt_tookit conversion is slated for cmd2 4.0.