nvim: after/ftplugin/cabal: Enable HLS on cabal files

Enabling HLS on cabal files allows us to format them using gq. HLS
just uses the internal cabal-fmt plugin.
https://haskell-language-server.readthedocs.io/en/latest/features.html#formatting

nvim/.config/nvim/after/ftplugin/cabal.lua

@@ -0,0 +1,4 @@
+local lsp_utils  = require('lsp-utils')
+local hls_config = lsp_utils.hls_config()
+
+vim.lsp.start(hls_config)

nvim/.config/nvim/lua/lsp-utils.lua

@@ -177,7 +177,12 @@ function M.hls_config()
     root_dir            = root_directory,
     filetypes           = { 'haskell' },
     single_file_support = true,
-    settings            = { haskell = { formattingProvider = 'ormolu' } },
+    settings            = {
+      haskell = {
+        formattingProvider      = 'ormolu'  ,
+        cabalFormattingProvider = 'cabalfmt',
+      }
+    },
   }
 end