Refactor how flag usage is generated

[FollowTheProcess]

• Provide a way of iterating over all flags
• Move flag usage block writing into command.go
• Command now uses the new usage mechanism

Summary

[codecov]

Codecov Report

❌ Patch coverage is 86.95652% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.13%. Comparing base (76e9cea) to head (e41756a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
command.go	87.87%	2 Missing and 2 partials ⚠️
internal/flag/set.go	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #188      +/-   ##
==========================================
- Coverage   87.74%   85.13%   -2.61%     
==========================================
  Files           8        8              
  Lines        1501     1507       +6     
==========================================
- Hits         1317     1283      -34     
- Misses        122      162      +40     
  Partials       62       62              

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