Support new Xcode 16 XCBuildConfiguration format

[johnrbent]

Resolves #852

Short description 📝

Adds support for changes to the XCBuildConfiguration representation of xcconfig files contained within a file-system synchronized root group.

Solution 📦

Instead of a single baseConfigurationReference, these xcconfig files are represented by a pair of:

• baseConfigurationReferenceAnchor: reference to the synced root group.
• baseConfigurationReferenceRelativePath: relative path to the file from the root group.

*config files within a PBXGroup are still represented by the baseConfigurationReference

Implementation 👩‍💻👨‍💻

• Add the two new optional properties to the XCBuildConfiguration type following existing patterns for baseConfigurationReference
• Include new unit test on the encoding as well as a fixture project with xcconfig files in a synced root group.
• I have another PR ready for XcodeGraph to adopt these changes that I'll put up once this is merged.

(This is my first contribution to the project. I'm happy to make changes if needed)

[dosubot]

Related Documentation

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[johnrbent]

I wasn't sure about this, but I had to change this to get the correct indentation locally in Xcode. Maybe someone can explain why it should be 2?