Add --format code that inserts new code cell using ipylab

[JasonWeill]

Problem

When the notebook author wants a generative AI model to generate code, the output is in markdown or raw (text) format, so the notebook author cannot run the generated code as they can run other code in their notebook.

Proposed Solution

Add a --format code format option. When the user runs a %%ai magic command with this option, Jupyter AI inserts a new code cell using ipylab that contains the source code produced.

Update the example notebooks and documentation to include information about --format code.

Additional context

The --format code option should apply a prompt template to ensure that only source code, with no plain-text explanations, get output. See #42.