I guess this could be a cross-platform format but you could also just store it as a sequence of commands

    echo "Hello, world!" > file
    mkdir -p dir/subdir
    echo -n "Content.\n" > dir/subfile
    ln -s "target path" link
    printf '#!/bin/sh\necho Howdy!' > script
    chmod +x script

Would be the same as what's in the readme:

    {
    "file": "Hello, world!",
    "dir": {
        "subfile": "Content.\n",
        "subdir": {}
    },
    "symlink": ["link", "target path"],
    "script": ["script", "#!/bin/sh\necho Howdy!"]
    }