Add support for C# as a language identifier in markdown preview code block syntax highlighting

[Logerfo]

Currently, as far as I am aware, the only accepted language identifiers for C# code blocks in the markdown preview are cs and csharp.
Other markdown previewers, like github and gitlab issue trackers, supports a third alternative: c#, as the following:

```csharp
public int M() => 2; //works everywhere
```

```c#
public int M() => 2; //doesn't work in vscode markdown preview
```

My feature is request is for vscode to allow the mentioned alternative identifier. I also noticed something strange: the c# identifier works inside the editor, but not in the preview. I'd call this an inconsistency.