New C# concept exercise: Stack

Hello!

As a total beginner, I first met the [`Stack`](https://learn.microsoft.com/it-it/dotnet/api/system.collections.generic.stack-1?view=net-7.0) data structure in the exercise [Matching Brackets](https://exercism.org/tracks/csharp/exercises/matching-brackets): I was wondering I we could introduce it with an _ad-hoc_ concept exercise in the C# path.

(Note: this issue is just a reminder, I will open a thread on the topic and discuss about it with the C# maintainers)