Add features for inheritance exercise

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

There are two features required:

- Verify that the constructor of the `Character` class uses the `protected` modifier.
- Verify that the various fields used (hit points, spell prepared and potion drunk) use the `private` modifier. i.e. all fields should be private

The boilerplate is [here](https://github.com/exercism/v3/blob/master/languages/csharp/exercises/concept/inheritance/Inheritance.cs).