Add Practice Exercise: `luhn`

[0xNeshi]

Towards https://github.com/exercism/cairo/issues/113

Problem specification: https://github.com/exercism/problem-specifications/tree/main/exercises/luhn

TODO:

• generate the exercise with ./bin/fetch-configlet && ./bin/configlet create --practice-exercise luhn
• check if any of the existing tracks (examples below) implement the exercise and you can reuse the implementation:
  • Go: https://github.com/exercism/go/tree/main/exercises/practice
  • Rust: https://github.com/exercism/rust/tree/main/exercises/practice
  • Python: https://github.com/exercism/python/tree/main/exercises/practice
• use exercise's canonical-data.json to see which test cases to implement: https://github.com/exercism/problem-specifications/blob/main/exercises/luhn/canonical-data.json

[0xNeshi]

Created a PR for this, reassigning to myself