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/languages/Cargo.toml

@@ -37,7 +37,6 @@ smol.workspace = true
 task.workspace = true
 toml.workspace = true
 tree-sitter-bash.workspace = true
-tree-sitter-c-sharp.workspace = true
 tree-sitter-c.workspace = true
 tree-sitter-clojure.workspace = true
 tree-sitter-cpp.workspace = true