Add positional and option set. Refactor formatters.

[roanutil]

• Add OptionSet
• Add Positional
• Fix typo in doc comment
• Update dependencies in Package.resolved
• Refactor formatters

[codecov]

Codecov Report

Merging #7 (4a27513) into main (7cff84a) will decrease coverage by 5.03%.
The diff coverage is 63.56%.

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
- Coverage   82.10%   77.07%   -5.03%     
==========================================
  Files          16       19       +3     
  Lines         961     1339     +378     
==========================================
+ Hits          789     1032     +243     
- Misses        172      307     +135     

Impacted Files	Coverage Δ
...ources/ArgumentEncoding/CommandRepresentable.swift	100.00% <ø> (ø)
Sources/ArgumentEncoding/Flag.swift	100.00% <ø> (ø)
...rgumentEncoding/TopLevelCommandRepresentable.swift	100.00% <ø> (ø)
Sources/ArgumentEncoding/PositionalArgument.swift	20.14% <20.14%> (ø)
Sources/ArgumentEncoding/OptionSet.swift	32.43% <32.43%> (ø)
Sources/ArgumentEncoding/Option.swift	43.44% <71.42%> (+4.73%)	⬆️
Tests/ArgumentEncodingTests/PositionalTests.swift	85.71% <85.71%> (ø)
Tests/ArgumentEncodingTests/OptionSetTests.swift	92.85% <92.85%> (ø)
Sources/ArgumentEncoding/Formatters.swift	96.36% <96.36%> (ø)
Sources/ArgumentEncoding/ArgumentGroup.swift	88.39% <100.00%> (+0.53%)	⬆️
... and 7 more