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 82.60870% with 4 lines in your changes missing coverage. Please review.

Project coverage is 81.35%. Comparing base (b2660f3) to head (8b992d3).

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   82.11%   81.35%   -0.77%     
==========================================
  Files          23       23              
  Lines        1118     1437     +319     
==========================================
+ Hits          918     1169     +251     
- Misses        200      268      +68     

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