“FinderSync keeps track of the windows that are open in the Finder and listens for changes in the filesystem.”
Is it possible for AppleScript to listen for changes in the filesystem?
Yes, with launchd triggering an AppleScript.
A folder action can only detect whether a file is added or removed,
but not both at the same time
For launchd, read Using launchd with AppleScript to Access a Flash Drive Automaticallyby Craig Smith