nvim: after/ftplugin/cabal: Add cabalfmt plugin

Since we reverted cabal-fmt support via HLS in the previous commit
add the cabalfmt plugin. Just add it to our own ftplugin instead of
adding via the plugin list.

nvim/.config/nvim/after/ftplugin/cabal/cabalfmt.vim

@@ -0,0 +1,55 @@
+" Taken from https://github.com/sdiehl/vim-cabalfmt
+
+if !exists("g:cabalfmt_command")
+  let g:cabalfmt_command = "cabal-fmt"
+endif
+
+if !exists("g:cabalfmt_options")
+  let g:cabalfmt_options = [""]
+endif
+
+function! s:OverwriteBuffer(output)
+  if &modifiable
+    let l:curw = winsaveview()
+    try | silent undojoin | catch | endtry
+    let splitted = split(a:output, '\n')
+    if line('$') > len(splitted)
+      execute len(splitted) .',$delete'
+    endif
+    call setline(1, splitted)
+    call winrestview(l:curw)
+  else
+    echom "Cannot write to non-modifiable buffer"
+  endif
+endfunction
+
+function! s:RunCabal()
+  if exists("bufname")
+    let output = system(g:cabalfmt_command . " " . join(g:cabalfmt_options, ' ') . " " . bufname("%"))
+  else
+    let stdin = join(getline(1, '$'), "\n")
+    let output = system(g:cabalfmt_command . " " . join(g:cabalfmt_options, ' '), stdin)
+  endif
+  if v:shell_error != 0
+    echom output
+  else
+    call s:OverwriteBuffer(output)
+    if exists("bufname")
+      write
+    endif
+  endif
+endfunction
+
+function! s:CabalHaskell()
+  if executable(g:cabalfmt_command)
+    call s:RunCabal()
+  else
+    echom "cabal-fmt executable not found"
+  endif
+endfunction
+
+function! RunCabal()
+  call s:CabalHaskell()
+endfunction
+
+nnoremap <buffer> <silent> gq :call RunCabal()<CR>