Add affine-cipher exercise

[JohnMertz]

Forum discussion located: https://forum.exercism.org/t/add-affine-cipher-exercise/15516

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[JohnMertz]

Note that there are 2 trailing commas in the config.json which trips the linter. I removed them from my branch, but don't want to open a new PR.

[kotp]

Note that there are 2 trailing commas in the config.json which trips the linter. I removed them from my branch, but don't want to open a new PR.

No need to create a new PR at this time. You can even do a git push -f origin affine-cipher if it fits in this patch, and was a mistake (I have not looked at the code specifically yet).