Add Sum of Multiples exercise

Hi! I’ve created the PR here.

Thank you!

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One of the tests wasn’t passing, the one involving macOS. Everything was fine here on my local machine and also on the other tests (involving Ubuntu). After a bit of fiddling here, I found out about the script to test against the docker image and finally could reproduce the error and fix it. Now all the tests pass.

I’m sorry for creating a second PR, I thought the commit would supersede the previous one. I’m still learning about how GitHub works.

Anyways, thank you again. Now that I know how, next time I’ll be able to test the example solution against the docker image before making a PR.