This PR extracts C# support into an extension and removes the built-in
C# support from Zed.
Tested using a Nix shell:
```
nix-shell -p dotnet-sdk omnisharp-roslyn
```
Release Notes:
- Removed built-in support for C#, in favor of making it available as an
extension. The C# extension will be suggested for download when you open
a `.cs` file.
