Motivation for this change

Closes #57099

As one would expect, this increases the closure size of vimPlugins.vim-stylish-haskell by a lot, from 3.1KB to 40.6MB, but this is worth it and makes sense.

Tested this by doing

$ cd /path/to/nixpkgs
$ nix-build -E 'with import ./. {}; neovim.override { configure = { packages.myVimPackage = with pkgs.vimPlugins; { start = [  vim-stylish-haskell ]; }; }; }'
$ export PATH=""
$ result/bin/nvim

Then opening a non-stylished Haskell file (such as https://github.com/mrkkrp/megaparsec/blob/master/Text/Megaparsec.hs) and hitting :w -> stylish does the styling.

Ping @lrworth

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.