Add features for flag-enums exercise

See the [flag-enums exercise](https://github.com/exercism/v3/blob/master/languages/csharp/exercises/concept/flag-enums/.meta/design.md).

Two features are required for this exercise:

- Verify that the `Permission` enum is marked with the `[Flags]` attribute.
- Suggest using `byte` as the enum's backing type if no backing type was explicitly specified.

The boilerplate is [here](https://github.com/exercism/v3/blob/master/languages/csharp/exercises/concept/flag-enums/FlagEnums.cs).
Example code is [here](https://github.com/exercism/v3/blob/master/languages/csharp/exercises/concept/flag-enums/.meta/Example.cs).