Folders support

[soffes]

It looks like folders aren't supported yet.

`PBXGroup` attempted to initialize an object with unknown ISA `PBXFileSystemSynchronizedRootGroup` from attributes: `{"isa"=>"PBXFileSystemSynchronizedRootGroup", "explicitFileTypes"=>{}, "explicitFolders"=>[], "path"=>"Shims", "sourceTree"=>"<group>"}` (RuntimeError)
If this ISA was generated by Xcode please file an issue: https://github.com/CocoaPods/Xcodeproj/issues/new

Maybe this is as easy and just bumping the version of xcodeproj 🤞

[keith]

looks like upstream does support CocoaPods/Xcodeproj#985, but i think we'd have to handle the new type specifically to be able to know that files are a part of it? not sure