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.
