Problem 45490. Luhn's Algorithm

Luhn's Algorithm is used as a checksum for credit card numbers or similar identifiers. It can detect single-digit changes and swapped digits. It consists of four steps:

1. Double every second digit (starting with the last one).
2. If a digit exceeds 9 due to the doubling, subtract 9.
3. Add all the digits.
4. Append the last digit of the sum to the original number.

More information can be found on https://en.wikipedia.org/wiki/Luhn_algorithm or in ISO/IEC 7812-1.

Your function takes the original numbers as a character vector, and it has to return a character vector that includes the checksum.