EZP-20389 - Implement old .tpl engine as a real Symfony template engine

This PR standardizes the legacy *eZ Template* engine as a real Symfony template engine, which allows to render the result of a controller in a legacy template (can be useful in the case of module migration).

Furthermore, it allows to include legacy templates directly with `include` Twig tag :

```jinja
{% include 'design:test/helloworld.tpl' with {'foo': 'bar'} %}
```

This replace the following:

```jinja
{% ez_legacy_include 'design:test/helloworld.tpl' with {'foo': 'bar'} %}
```

Usage of `include` is more transparent and avoids confusion for developers.