Remove dependency on Python's built-in cmd module

[tleonhardt]

The cmd2.Cmd class no longer inherits from cmd.Cmd and the cmd2 module no longer depends on Python's built-in cmd module.

To make this happen we had to do the following:

• Add an extra parameter to cmd2.Cmd.__init__ and several extra initializations within it
• Add implementation for the completedefault and completenames methods
• Eliminate some redundant casts to fix mypy warnings

[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 (35af636) to head (c05a16d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1539      +/-   ##
==========================================
- Coverage   98.94%   98.92%   -0.02%     
==========================================
  Files          21       21              
  Lines        4933     4942       +9     
==========================================
+ Hits         4881     4889       +8     
- Misses         52       53       +1     

Flag	Coverage Δ
unittests	98.92% <100.00%> (-0.02%)	⬇️

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[github-actions]

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

[tleonhardt]

It's complaining about code coverage and the reason I think is we aren't covering the line for the case where stdout is not None in the cmd2.Cmd.__init__.