refactor: move optional modules to modules/optional/

- Move virtualbox-guest.nix and vscode-server.nix from modules/server/ to modules/optional/
- Create modules/optional/README.md with usage docs
- Update profiles/virtualbox.nix and profiles/wsl.nix import paths
- Update modules/server/README.md to reflect removal
- Update root README.md directory tree with optional/ entry

modules/optional/vscode-server.nix

@@ -0,0 +1,13 @@
+{ inputs, ... }:
+{
+  imports = [
+    inputs.vscode-server.nixosModules.default
+  ];
+
+  services.vscode-server.enable = true;
+
+  programs.direnv.enable = true;
+  
+  # 允许未打补丁的动态二进制文件在 NixOS 上运行
+  programs.nix-ld.enable = true;
+}