[Neovim] Adds Sidekick plugin.

home-manager/features/neovim/default.nix

@@ -21,6 +21,7 @@
       sqlite # For Snacks
       imagemagick # For image conversion/display
       vscode-langservers-extracted # For language servers (it wants this version of eslint for some reason)
+      copilot-language-server # Enable the copilot language server
     ];
   };