languages/haskell.nix: address PR comments

- Rename hls -> haskell-language-server to match the nixpkgs name
- Remove unneeded injection
- Only set lsp filetypes in the language module, not the preset
- Remove unneeded `mkDefault`
- Use `root_markers` instead of yucky inline Lua

modules/plugins/lsp/presets/default.nix

@@ -22,7 +22,7 @@
     ./gopls.nix
     ./harper.nix
     ./helm-ls.nix
-    ./hls.nix
+    ./haskell-language-server.nix
     ./intelephense.nix
     ./jdt-language-server.nix
     ./jinja-lsp.nix