Extract C# support into an extension (#9971)

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.

crates/extensions_ui/src/extension_suggest.rs

@@ -16,6 +16,7 @@ fn suggested_extensions() -> &'static HashMap<&'static str, Arc<str>> {
         [
             ("astro", "astro"),
             ("beancount", "beancount"),
+            ("csharp", "cs"),
             ("dockerfile", "Dockerfile"),
             ("elisp", "el"),
             ("fish", "fish"),