renamed changed lsp names and use web-devicons as a full plugin (#49)

fix #48 

- Renamed LSP servers as they were changed upstream.
- Enable web-devicons as a nixvim plugin instead of installing them as a
extra package.

config/plugins/utils/extra_plugins.nix

@@ -1,5 +1,4 @@
 {pkgs, ...}: {
   extraPlugins = with pkgs.vimPlugins; [
-    nvim-web-devicons
   ];
 }