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.
Add a data transfer object that stores the source file name, the file
name to generate and an optional path to create for each file to
generate.
As I'm using a `readonly` class for the DTO, this required updating PHP
to 8.2.
nix develop --command php bin/build-configs run