Add sum of multiples exercise

[oxe-i]

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[github-actions]

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[oxe-i]

I'm not entirely sure why the code isn't passing the test for macOS. Did some research and it might have something to do with stack alignment.

[oxe-i]

No, it's not, I should've used r8 instead of r9 to move into rcx in end_extraction. Somehow this slipped through. It's a bit mysterious how the solution passed (and keeps passing) all the tests in Linux environments, both on my local machine and here.

The upside here is that I learned about the existence of ./bin/verify-exercises-in-docker and I'm now able to test against docker image in the future before making a PR.