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/ls.feature

@@ -0,0 +1,15 @@
+Feature: ls
+  In order to see the directory structure
+  As a UNIX user
+  I need to be able to list the current directory's contents
+
+  Scenario: List 2 files in a directory
+    Given I am in a directory "test"
+    And I have a file named "foo"
+    And I have a file named "bar"
+    When I run "ls"
+    Then I should get:
+      """
+      bar
+      foo
+      """