Use argument parsers to run AI commands

[JasonWeill]

Problem

This is a follow-up to the work done on #100. This PR adds an _ai_command function that uses the arguments provided as a space-delimited list. @dlqqq writes:

It would be better to leverage argparse.add_subparser here, but that would require us to rip out the IPython decorators and create the parser object ourselves. [The IPython.core.magic_arguments module]](/~https://github.com/ipython/ipython/blob/main/IPython/core/magic_arguments.py) does not [have] a decorator for this functionality. In the long-term, it would be more robust to use argparse directly rather than relying on the intermediary layer provided by IPython.

Proposed Solution

Use argparse.add_subparser to parse arguments to %ai commands. As an alternative, consider click; argparse is somewhat notorious for changing its functionality across Python versions, so it may not be ideal for us.