Initialize Positional with a type that conforms to ArgumentGroup

[roanutil]

If a command A takes command B as an argument, this makes it easier to define command B as a Positional property on command A.

For example:
mise --cd {{ project path }} x tuist -- tuist version
mise is command A and tuist version is command B

[codecov]

Codecov Report

Attention: Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.

Project coverage is 81.35%. Comparing base (0ec6b15) to head (f84d612).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/ArgumentEncoding/PositionalArgument.swift	63.63%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   81.38%   81.35%   -0.04%     
==========================================
  Files          23       23              
  Lines        1418     1437      +19     
==========================================
+ Hits         1154     1169      +15     
- Misses        264      268       +4     

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