Implement Xcode 16 file system synchronized directories

[tomlokhorst]

Fixes #914 and #252

• This finds all folders that are synced to the file system.
• They can be at the top level, or deeper in the Xcode group hierarchy.
• All those folders are scanned for files that need to become Rswift resources.
• Source code files (like .swift, .c, .m, etc. are excluded).
• Membership exceptions are implemented. A file can be either excluded from a target, or included from a different target.
• Localised membership exceptions for inclusion are also implemented (e.g. a .storyboard file with .strings file next to it for localisation).