Extract Haskell support into an extension (#9814)

This PR extracts Haskell support into an extension and removes the
built-in Haskell support from Zed.

I tested out the extension locally in a Nix shell using `nix-shell -p
ghc haskell-language-server` to confirm the language server still
operated as expected:

<img width="341" alt="Screenshot 2024-03-26 at 11 26 26 AM"
src="https://github.com/zed-industries/zed/assets/1486634/df16fd38-4046-4a45-ac9f-c2b85bffe5c0">

Release Notes:

- Removed built-in support for Haskell, in favor of making it available
as an extension. The Haskell extension will be suggested for download
when you open a `.hs` file.

extensions/haskell/src/haskell.rs

@@ -0,0 +1,27 @@
+use zed_extension_api::{self as zed, Result};
+
+struct HaskellExtension;
+
+impl zed::Extension for HaskellExtension {
+    fn new() -> Self {
+        Self
+    }
+
+    fn language_server_command(
+        &mut self,
+        _config: zed::LanguageServerConfig,
+        worktree: &zed::Worktree,
+    ) -> Result<zed::Command> {
+        let path = worktree
+            .which("haskell-language-server-wrapper")
+            .ok_or_else(|| "hls must be installed via ghcup".to_string())?;
+
+        Ok(zed::Command {
+            command: path,
+            args: vec!["lsp".to_string()],
+            env: Default::default(),
+        })
+    }
+}
+
+zed::register_extension!(HaskellExtension);