Add Behat

Add Behat as a dev dependency and add the example `ls` example.

Due to a deprecation error, I needed to downgrade to PHP 8.1 on stream,
though this would error the main `app:generate` command as I'm using
`readonly` classes that were introduced in PHP 8.2.

flake.nix

@@ -1,32 +1,24 @@
 # Do not edit this file. It is automatically generated by https://www.oliverdavies.uk/build-configs.
+
 {
   inputs = {
     devshell.url = "github:numtide/devshell";
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
   };
 
-  outputs = inputs @ {flake-parts, ...}:
-    flake-parts.lib.mkFlake {inherit inputs;} {
-      imports = [inputs.devshell.flakeModule];
+  outputs = inputs@{ flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      imports = [ inputs.devshell.flakeModule ];
 
-      systems = ["x86_64-linux"];
+      systems = [ "x86_64-linux" ];
 
-      perSystem = {
-        config,
-        self',
-        inputs',
-        pkgs,
-        system,
-        ...
-      }: {
+      perSystem = { config, self', inputs', pkgs, system, ... }: {
         devshells.default = {
           packages = with pkgs; [
-            "php82"
-            "php82Packages.composer"
+            "php81"
+            "php81Packages.composer"
           ];
         };
-
-        formatter = pkgs.alejandra;
       };
     };
 }