Add support for sendable conformance argument

[nolinmcfarland]

📝 Summary

Added support for sendable conformance (.checked | .unchecked) on generated mocks. This is provided as an argument on the @Mocked macro:

• Passing .checked (also the Default) is a no-op right now since the mock's sendability is inherited through the implementation and implicitly checked by the compiler already.
• Passing .unchecked conforms the generated mock to @unchecked Sendable, supporting a fix for the issue described in [Feature] Support unchecked Sendable #86.

To accomplish this, this pull request:

• Adds a new SendableConformance macro argument.
• Defines a new MacroArgument protocol and refactors argument parsing to allow for partial arguments (ex., having sendableConformance but not compilationCondition), giving us greater flexibility for future arguments.
• Makes @unchecked Sendable a reusable extension of InheritedTypeSyntax.
• Adds @unchecked Sendable to the inheritance clause when .unchecked is passed in for the sendableConformance argument.
• Updates tests and documentation.

🛠️ 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?

I replicated the scenario and error described in #86 and verified it is resolved when conforming the mock to @unchecked Sendable. Unit tests verifying the macro's expansion passed. Please see screenshots of the expansions below:

Sendable Conformance	Macro Expansion
Default (.checked)
.checked
.unchecked

🔗 Related PRs or Issues

• Closes [Feature] Support unchecked Sendable #86

✅ 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

All modified and coverable lines are covered by tests ✅

Project coverage is 68.77%. Comparing base (3970815) to head (28fe528).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
+ Coverage   68.64%   68.77%   +0.13%     
==========================================
  Files          69       71       +2     
  Lines        4538     4557      +19     
==========================================
+ Hits         3115     3134      +19     
  Misses       1423     1423              

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