neovim: Add fcitx-vim

This plugin enables smooth integration between fcitx5 and neovim. In
order words, you no longer have to manually change input methods when
switching between normal mode and insert mode.

modules/neovim/default.nix

@@ -263,6 +263,7 @@ in
           bufdelete-nvim
           vim-crystal
           vim-nix-rummik
+          fcitx-vim
         ];
       };