Refactor MacroArgumentValue, reorganize files, and add unit tests

[graycampbell]

📝 Summary

• Refactored MacroArgumentValue to make the initializer throw an error instead of returning nil.
• Updated MockCompilationCondition and MockSendableConformance to work with changes to MacroArgumentValue.
• Added new unit tests and updated existing unit tests.

🛠️ Type of Change

• Bug fix (change that fixes an issue)
• New feature (change that adds functionality)
• Breaking change (bug fix or feature that is not backwards compatible)
• Documentation (DocC, API docs, markdown files, templates, etc.)
• Testing (new tests, updated tests, etc.)
• Refactoring or code formatting (no logic changes)
• Updating dependencies (Swift packages, Homebrew, etc.)
• CI/CD (change to automated workflows)

🧪 How Has This Been Tested?

• New unit tests.
• Existing unit tests still pass.

✅ Checklist

• I have added relevant tests
• I have verified all tests pass
• I have formatted my code using SwiftFormat
• I have updated documentation (if needed)
• I have added the appropriate label to my PR
• I have read the contributing guidelines
• I agree to follow this project's Code of Conduct

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.

Project coverage is 69.24%. Comparing base (ed08a0c) to head (a900cc4).

Files with missing lines	Patch %	Lines
...ompilationCondition/MockCompilationCondition.swift	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
+ Coverage   68.87%   69.24%   +0.37%     
==========================================
  Files          71       73       +2     
  Lines        4572     4585      +13     
==========================================
+ Hits         3149     3175      +26     
+ Misses       1423     1410      -13     

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