feat: Introduce the new Xcode 16 models PBXFileSystemSynchronizedRootGroup and PBXFileSystemSynchronizedBuildFileExceptionSet

[pepicrft]

Apple is introducing two new models, PBXFileSystemSynchronizedRootGroup and PBXFileSystemSynchronizedBuildFileExceptionSet, to reduce Git conflicts. Those types allow targets to reference a folder, whose files get automatically synchronized instead of creating references for every single file.
This PR adds those models along with the serialization logic. Once we use them from Tuist, I'll get ideas for convenient APIs that we might consider pushing down to the XcodeProj layer.

[kwridan]

Thanks for adding this @pepicrft

Worth adding a fixture to test out reading / writing.

From a quick test it seems there might be a few missing integrations to wire up the new models up as it still doesn't recognise them when attempt to read an Xcode 16 project that uses the new folder references.

failed: caught error: "The element PBXFileSystemSynchronizedRootGroup is not supported."

[pepicrft]

@kwridan I uncovered the missing pieces through tests and fixed some minor issues. I'd appreciate a look when you have some spare time.

[kwridan]

thanks for the updates @pepicrft - XcodeProj now successfully reads / write a sample Xcode 16 project 🎉

I spotted a few missing bits from testing that are cause diffs after a read / write.

[pepicrft]

@kwridan this should be ready for another look.

[kwridan]

Awesome thank you @pepicrft 🎉

There are a few additional properties that can be set but can be done as a follow up:

• compilerFlags
• platformFilters

[pepicrft]

There are a few additional properties that can be set but can be done as a follow up:

I'll add those as part of this PR. Thanks for the review @kwridan

[pepicrft]

@kwridan could you remind me of where platform filters are used? I also noticed that there's another object type to define exception for files referenced by copy build phases. I created an issue to tackle that in a follow-up PR.