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.

extensions/csharp/src/csharp.rs

@@ -0,0 +1,116 @@
+use std::fs;
+use zed_extension_api::{self as zed, Result};
+
+struct CsharpExtension {
+    cached_binary_path: Option<String>,
+}
+
+impl CsharpExtension {
+    fn language_server_binary_path(
+        &mut self,
+        config: zed::LanguageServerConfig,
+        worktree: &zed::Worktree,
+    ) -> Result<String> {
+        if let Some(path) = &self.cached_binary_path {
+            if fs::metadata(path).map_or(false, |stat| stat.is_file()) {
+                return Ok(path.clone());
+            }
+        }
+
+        if let Some(path) = worktree.which("OmniSharp") {
+            self.cached_binary_path = Some(path.clone());
+            return Ok(path);
+        }
+
+        zed::set_language_server_installation_status(
+            &config.name,
+            &zed::LanguageServerInstallationStatus::CheckingForUpdate,
+        );
+        let release = zed::latest_github_release(
+            "OmniSharp/omnisharp-roslyn",
+            zed::GithubReleaseOptions {
+                require_assets: true,
+                pre_release: false,
+            },
+        )?;
+
+        let (platform, arch) = zed::current_platform();
+        let asset_name = format!(
+            "omnisharp-{os}-{arch}-net6.0.{extension}",
+            os = match platform {
+                zed::Os::Mac => "osx",
+                zed::Os::Linux => "linux",
+                zed::Os::Windows => "win",
+            },
+            arch = match arch {
+                zed::Architecture::Aarch64 => "arm64",
+                zed::Architecture::X86 => "x86",
+                zed::Architecture::X8664 => "x64",
+            },
+            extension = match platform {
+                zed::Os::Mac | zed::Os::Linux => "tar.gz",
+                zed::Os::Windows => "zip",
+            }
+        );
+
+        let asset = release
+            .assets
+            .iter()
+            .find(|asset| asset.name == asset_name)
+            .ok_or_else(|| format!("no asset found matching {:?}", asset_name))?;
+
+        let version_dir = format!("omnisharp-{}", release.version);
+        let binary_path = format!("{version_dir}/OmniSharp");
+
+        if !fs::metadata(&binary_path).map_or(false, |stat| stat.is_file()) {
+            zed::set_language_server_installation_status(
+                &config.name,
+                &zed::LanguageServerInstallationStatus::Downloading,
+            );
+
+            zed::download_file(
+                &asset.download_url,
+                &version_dir,
+                match platform {
+                    zed::Os::Mac | zed::Os::Linux => zed::DownloadedFileType::GzipTar,
+                    zed::Os::Windows => zed::DownloadedFileType::Zip,
+                },
+            )
+            .map_err(|e| format!("failed to download file: {e}"))?;
+
+            let entries =
+                fs::read_dir(".").map_err(|e| format!("failed to list working directory {e}"))?;
+            for entry in entries {
+                let entry = entry.map_err(|e| format!("failed to load directory entry {e}"))?;
+                if entry.file_name().to_str() != Some(&version_dir) {
+                    fs::remove_dir_all(&entry.path()).ok();
+                }
+            }
+        }
+
+        self.cached_binary_path = Some(binary_path.clone());
+        Ok(binary_path)
+    }
+}
+
+impl zed::Extension for CsharpExtension {
+    fn new() -> Self {
+        Self {
+            cached_binary_path: None,
+        }
+    }
+
+    fn language_server_command(
+        &mut self,
+        config: zed::LanguageServerConfig,
+        worktree: &zed::Worktree,
+    ) -> Result<zed::Command> {
+        Ok(zed::Command {
+            command: self.language_server_binary_path(config, worktree)?,
+            args: vec!["-lsp".to_string()],
+            env: Default::default(),
+        })
+    }
+}
+
+zed::register_extension!(CsharpExtension);