Improve Concept Exercise: attack-of-the-trolls/flag-enums

[valentin-p]

This issue describes how to improve the attack-of-the-trolls concept exercise for the concept flag-enums for the C# track.

Getting started

Please please please read the docs before starting. Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time. So, before creating the issue, please read up on the following documents:

• The features of v3.
• Rationale for v3.
• What are concept exercise and how they are structured?

Please also watch the following video:

• The Anatomy of a Concept Exercise.

Goal

The Enums Advanced exercise should be lightly modified to require parsing an enum with Enum.Parse.
Consider adding the generic-types Concept as prerequisites if necessary (edit config.json and .meta/design.md).
Change the solution and test file and the .meta/Example.cs accordingly
Change the hints.md and instructions.md files accordingly

Help

If you have any questions while improving the exercise, please post the questions as comments in this issue.

[valentin-p]

Closes exercism/v3#663