Delete plugins directory

[tleonhardt]

Delete plugins directory as this felt outdated and not really relevant within the cmd2 repo itself.

Also:

• Update documentation on using mixins with cmd2
• Add a mixin.py example
• Move ruff config to ruff.toml (from pyproject.toml
• Upgrade to ruff to 0.13.1 (from 0.13.0)

[github-actions]

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.92%. Comparing base (8a5d1bc) to head (5650de5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1523      +/-   ##
==========================================
+ Coverage   98.90%   98.92%   +0.02%     
==========================================
  Files          21       21              
  Lines        4933     4933              
==========================================
+ Hits         4879     4880       +1     
+ Misses         54       53       -1     

Flag	Coverage Δ
unittests	98.92% <ø> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[github-actions]

📋 Review Summary

This pull request is a nice cleanup and refactoring. It removes the outdated plugins directory and replaces it with clearer documentation and examples for using mixins to extend cmd2. The move of the ruff configuration to its own file is also a good improvement.

🔍 General Feedback

• The new documentation for mixins is well-written and helpful.
• The example code is clear and demonstrates the concepts well.
• The project structure is cleaner with the removal of the plugins directory.