chore: add flake for development with Nix (#5509)

flake.nix

@@ -0,0 +1,25 @@
+{
+  description = "code-server";
+
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem
+      (system:
+        let pkgs = nixpkgs.legacyPackages.${system};
+            nodejs = pkgs.nodejs-16_x;
+            yarn' = pkgs.yarn.override { inherit nodejs; };
+        in {
+          devShells.default = pkgs.mkShell {
+            nativeBuildInputs = with pkgs; [
+              nodejs yarn' python pkg-config git rsync jq moreutils
+            ];
+            buildInputs = with pkgs; (lib.optionals (!stdenv.isDarwin) [ libsecret ]
+                          ++ (with xorg; [ libX11 libxkbfile ])
+                          ++ lib.optionals stdenv.isDarwin [
+                            AppKit Cocoa CoreServices Security cctools xcbuild
+                          ]);
+          };
+        }
+      );
+}