Add File system synchronized groups comparator

Resolves: bloomberg#140

- Adding a new comparator to diff the Xcode 16 synchronized groups which can include sources

Test Plan:

Run xcdiff on the new fixture and verify it flags diffs for the mismatching groups

```
swift run xcdiff -p1 Fixtures/xcode16_syncgroups_1/Project.xcodeproj -p2 Fixtures/xcode16_syncgroups_2/Project.xcodeproj -v
```

Signed-off-by: Kassem Wridan <[email protected]>