Added get_type_hints cache; documentation updates

[dskrypa]

• Added lru_cache for get_type_hints for type annotation detection since get_type_hints returns a dict of {attr_name: annotation} for all attributes in the given class, and re-builds that dict each time it is called. Since multiple attributes are examined in each class that extends Command, caching the result improves performance for that step.
• Updated docs to include info about AsyncCommand
• Updated docs to suggest using UsageError and BadArgument when manually validating user input

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.43%. Comparing base (aa3da9d) to head (7de4769).

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #61   +/-   ##
=======================================
  Coverage   98.43%   98.43%           
=======================================
  Files          49       49           
  Lines        6628     6630    +2     
  Branches     1396     1396           
=======================================
+ Hits         6524     6526    +2     
  Misses         95       95           
  Partials        9        9           

Flag	Coverage Δ
unittests	98.43% <100.00%> (+<0.01%)	⬆️

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