Enhanced tracebacks

Tracebacks are enhanced by default, screenshot with monokai theme:

Test your theme with cli2-traceback command!

Traceback hook with more context lines, locals, colors, and line numbers.

Having more context lines and locals is useful to set context for LLMs, colors are just cool. Line numbers help pinpoint errors precisely.

Based on cli2.theme, you can override the color palette with environment variables.