Add knapsack exercise

[JohnMertz]

This is my first crack at trying to contribute an exercise, so please feel free to point me in the right direction if I've done something incorrectly. I read the docs and tried to make the commit as similar as possible to other exercise PRs.

If all goes well, I'll be happy to create more, for want of having any more to do! :)

The discussion is located here.

[github-actions]

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