Handle sync conflicts without data loss

[claucambra]

At present, the macOS virtual files client implements a very simple form of conflict handling: the latest remote changes are down-synced, replacing local changes. Conversely, the latest local changes will always replace the file's state on the server. Your task is to implement sync conflict handling that avoids data loss. A successful submission of this task will demonstrate handling of sync conflicts that provide users with safe and clear approaches with which to handle sync conflicts, avoiding data loss.