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 98.94%. Comparing base (35af636) to head (340b688).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1539   +/-   ##
=======================================
  Coverage   98.94%   98.94%           
=======================================
  Files          21       21           
  Lines        4933     4945   +12     
=======================================
+ Hits         4881     4893   +12     
  Misses         52       52           

Flag	Coverage Δ
unittests	98.94% <100.00%> (+<0.01%)	⬆️

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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__.