Add CSharp language and refine Nix LSP settings

- Define CSharp language with format_on_save, tab_size, and servers
- Expand Nix language configuration with formatter and tab settings
- Simplify nixd LSP config by removing redundant initialization options

modules/core/packages/appimages/helium.nix

@@ -37,3 +37,4 @@ in
     };
   }
 # https://github.com/imputnet/helium-linux/releases/download/0.11.7.1/helium-0.11.7.1-x86_64.AppImage
+