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.

features/bootstrap/FeatureContext.php

@@ -0,0 +1,64 @@
+<?php
+
+use Behat\Behat\Context\Context;
+use Behat\Gherkin\Node\PyStringNode;
+use Behat\Gherkin\Node\TableNode;
+
+/**
+ * Defines application features from the specific context.
+ */
+class FeatureContext implements Context
+{
+    /**
+     * Initializes context.
+     *
+     * Every scenario gets its own context instance.
+     * You can also pass arbitrary arguments to the
+     * context constructor through behat.yml.
+     */
+    public function __construct()
+    {
+    }
+
+    /**
+     * @Given I am in a directory :arg1
+     */
+    public function iAmInADirectory($dir)
+    {
+        if (!file_exists($dir)) {
+            mkdir($dir);
+        }
+
+        chdir($dir);
+    }
+
+    /**
+     * @Given I have a file named :arg1
+     */
+    public function iHaveAFileNamed($file)
+    {
+        touch($file);
+    }
+
+    /**
+     * @When I run :arg1
+     */
+    public function iRun($command)
+    {
+        exec($command, $output);
+
+        $this->output = trim(implode("\n", $output));
+    }
+
+    /**
+     * @Then I should get:
+     */
+    public function iShouldGet(PyStringNode $string)
+    {
+        if ((string) $string !== $this->output) {
+            throw new Exception(
+                "Actual output is:\n" . $this->output
+            );
+        }
+    }
+}