Add flake for contract development (#11)

- Works for `yarn install`, `yarn build:all`, `forg test`
- The command `yarn test` fails with

  1) ArbRollupFastConfirm
       should initialize:
     Error: missing argument:  in Contract constructor
     (count=0, expectedCount=1, code=MISSING_ARGUMENT, version=contracts/5.7.0)

flake.nix

@@ -0,0 +1,39 @@
+{
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  inputs.utils.url = "github:numtide/flake-utils";
+  inputs.foundry.url = "github:shazow/foundry.nix/monthly"; # Use monthly branch for permanent releases
+
+  outputs = { self, nixpkgs, utils, foundry }:
+    utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs
+          {
+            inherit system;
+            overlays = [
+              foundry.overlay
+              (final: prev: {
+                # Overlaying nodejs here to ensure nodePackages use the desired
+                # version of nodejs use by the upstream CI.
+                nodejs = prev.nodejs-18_x;
+                pnpm = prev.nodePackages.pnpm;
+                yarn = prev.nodePackages.yarn;
+              })
+            ];
+          };
+      in
+      {
+        devShells.default = with pkgs; mkShell {
+          buildInputs = [
+            foundry-bin
+            nodejs
+            yarn
+          ];
+          shellHook = ''
+            # Add node executables (incl. hardhat) to PATH
+            export PATH=$PWD/node_modules/.bin:$PATH
+          '';
+
+        };
+
+      });
+}