Support SerializedFile v26 (Unity 6.7)

Unity 6.6 is v23 of the file format.
At end of alpha Unity 6.7 has reached v26, with several significant changes.

    kIndependentTypeTreeVersion = 24, // 6000.7: TypeTree blobs are now versioned by their own TypeTreeFormatVersion instead of this enum; TypeTree format changes no longer bump the serialized file version.
    kSerializeReferenceRegistryFirst = 25, // 6000.7: the [SerializeReference] registry is a leading frame in the object's data (v3 table format) instead of a trailing TypeTree-described node. Older readers would misparse the frame bytes as field data, so they must reject the file on version.
    kSharedSubtreeSupport = 26, // Shared sub-typetree referencing: the file carries a table of compound subtrees stored once, keyed by content hash, that its types reference

This means the "dump", "analyze" commands will not work with the output from Unity 6.7

Most subcommands of the "serialized-file" will also not work (but the "header" command does work, that can be used to see the serialized file version)

Work includes:

• Updating to new snapshot UnityFileSystemApi dll (windows/mac/linux), e.g. take a snapshot from an official build of Unity 6.7 beta.
• Expose the API changes in UnityFileSystemApi through the C# wrapper
• Change the parsing code used by serialized-file command to reflect the updated file format (but continue to support reading older files)
• Add some reference files for automated tests
  • e.g. rebuild the UnityProjects/LeadingEdge project with Unity 6.7)
  • ideally add refernce files that demonstrate any new features and add specific tests for how this data is exposed
• Make sure docs / comments are updated to reflect the changes, e.g. any docs about TypeTrees should be revisited, and the serialized reference registry change may also influence the example.
• Update UnityDataTools version and create a release for download (along with other fixes).

Start with the serialized-file command parsing code and the existing UnityFileSystemApi C# wrapper, then inspect the Unity 6.7 beta snapshot DLLs for the listed API and file-format changes. Rebuild UnityProjects/LeadingEdge with Unity 6.7 to obtain reference files, add coverage for the new data while preserving older-file support, and update the mentioned TypeTree documentation and comments.