Support Xcode plist file format for project.pbxproj file

[mcumming]

This change will add support for the Xcode plist file format to the project.pbxproj file. While still drafting the functionality, the code will live SxS with the existing functionality. Once complete, the new feature will supplant the old json support.

It is quite possible the new support could end up in the Xamarin.MacDev repository for broader usage.

Microsoft Reviewers: Open in CodeFlow

[github-actions]

⚠️ Your code has been reformatted. ⚠️

If this is not desired, add the actions-disable-autoformat label, and revert the reformatting commit.

If files unrelated to your change were modified, try reverting the reformatting commit + merging with the target branch (and push those changes).