Add "D&D Character" practice exercise

[jesse-kroon]

As per the #48in24 challenge implementation status page.

[github-actions]

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[andrerfcsantos]

This is great work, thanks for powering it through!

Did a first pass review.

[andrerfcsantos]

Let's get this live! 🎉

Did some small changes to the tests since your last modifications. After all the work you did I didn't want to bore you with more work. The changes I've made:

• Made code that would generate scores for abilities slightly off the ranges, and noticed that just 100 iterations were not enough to reliably catch those cases. Ran the tests several times and it was somewhat easy to get tests passing when they shouldn't. Changed to 1000 iterations and it was a lot harder to get tests passing with the same wrong code.
• In TestModifier, changed the failure message to print tc.input.Score instead of tc.input. In case of failures, it was printing the message as Modifier({3}) = -3, want -4, now prints Modifier(3) = -3, want -4.
• Changed the order of benchmarks of each function to match the order by which the students have to implement the functions being benchmarked. Go runs the benchmarks by the order they are declared, and it was weird to see BenchmarkModifier be the last benchmark when Modifier is the first function students should implement.

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@jesse-kroon Thanks again for doing this and for pushing it through despite all the hurdles.

[jesse-kroon]

Thanks for the final edits @andrerfcsantos ! Good stuff.