feat: add optional debugAsWhichUser to XCScheme.LaunchAction

[macblazer]

Short description 📝

As a developer I want to be able to control the "Debug Process as..." setting within the Run action of a scheme so that I can easily construct projects that will launch scripts as root or test multi-user macOS apps.

Solution 📦

I first created a proof-of-concept scheme and used Xcode's GUI to set the "Debug Process as.." setting to "root". Then I looked at the resulting .xcscheme file and saw that it used an XML property named "debugAsWhichUser". I added that property to the XCScheme.LaunchAction type which models the Run action from the scheme file.

Implementation 👩‍💻👨‍💻

Detail in a checklist the steps that you took to implement the PR.

• Added the optional property.
• Added a fixture .xcscheme file to the unit tests that is copied from Xcode's GUI creation of schemes.
• Added unit tests showing the XCScheme.LaunchAction.debugAsWhichUser property is both read from and written to .xcscheme files.
• All other unit tests pass with no changes.

[fortmarek]

Thanks! Sorry for taking longer to review this one 🙏