Require mbstring extension for PHP in composer

mpdf requires the mbstring library per:
vendor/mpdf/mpdf/mpdf.php:if (!function_exists("mb_strlen")) { die("Error - mPDF requires mb_string functions. Ensure that PHP is compiled with php_mbstring.dll enabled."); }

It would be nice if composer required this so that one can find out on setup instead of using the library.

Thanks in advance!