Fix line ending sequence issue in checksums

[caendesilva]

Fix line ending sequence issue in checksums by creating a wrapper for the checksum call to replace all (*most) line ending sequences with LFs to prevent the issue raised in #85.

When running comparisons for the file cache, use FileCacheService::unixsum instead of md5 and FileCacheService::unixsumFile instead of md5_file to ensure compatibility.

public static function unixsum(string $string): string
{
    $string = str_replace(["\r\n", "\r"], "\n", $string);
    return md5($string);
}

public static function unixsumFile(string $file): string
{
    return static::unixsum(file_get_contents($file));
}