Add Flags Attribute concept exercise

[blackk-foxx]

Introduce the Flags Attribute (aka flags-discriminated-union) concept and provide an exercise to teach it. The exercise goal is to use the Flags attribute to build a password checker capable of reporting multiple rule violations in one call.

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[ErikSchierboom]

I'm currently away, so I don't have time to review this right now. Regarding your question: I'm positive that we have other exercises that have the same problem. Likely the discriminated unions exercises. Maybe you could check to see if that is referenced somewhere? There might be an exclude list or something.