nix: create initial flake

devShells/flake-module.nix

@@ -0,0 +1,17 @@
+{
+  perSystem =
+    {
+      pkgs,
+      ...
+    }:
+    {
+      devShells.default = pkgs.mkShell {
+        packages = [
+          pkgs.nil
+          pkgs.nixfmt-rfc-style
+          pkgs.cargo
+          pkgs.rust-analyzer
+        ];
+      };
+    };
+}