Add "Knapsack" exercise

[jesse-kroon]

No description provided.

[github-actions]

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[andrerfcsantos]

Let's get this exercise added!

Since it's been a while since we last talked and to make you not have to revisit this, I made a couple of tweaks in your branch:

• Made Item public. Usually types given to the student in the track are always public by default.
• Ran configlet sync for the exercise, which separated the instructions into instructions.md and introduction.md.
• Added a benchmark function. All practice exercises have this.
• Increased the difficulty of the exercise to 6, to make it in line with other dynamic programming exercises.
• Tweaked the comment in the function given to the student

Thanks a lot for all the work!