Number column index to string
var_dump(\Vtiful\Kernel\Excel::stringFromColumnIndex(0)); // A
var_dump(\Vtiful\Kernel\Excel::stringFromColumnIndex(28)); // AC
String column index to number
var_dump(\Vtiful\Kernel\Excel::columnIndexFromString('A')); // 0
var_dump(\Vtiful\Kernel\Excel::columnIndexFromString('AC')); // 28
