Improve file signature

`filepack archive`, added in #80, uses `FILEPACK` as the archive file signature. I think the file signature could be improved in a few ways: - Use an invalid UTF-8 sequence, to make it impossible for a plain text file to start with the file signature. - Use emoji, since as far as I know, there are no widely used file signatures that contain emoji - Add a version number - Make it longer/more random, so it's unlikely to appear by accident