Check to see if the user has im installed.

def has_im():
    """Check to see if the user has im installed."""
    if shutil.which("magick"):
        return ["magick", "convert"]

    if shutil.which("convert"):
        return ["convert"]

    logging.error("Imagemagick wasn't found on your system.")
    logging.error("Try another backend. (wal --backend)")
    sys.exit(1)