Twig slots

[m-vo]

This introduces a new concept to our Twig ecosystem called slots. It basically allows to define placeholder sections in your templates which can be enumerated and populated at runtime. For this, we analyze the node tree at template compile time and cache the respective information for our Inspector service to use. Slots are a new language construct and as such they support some syntactic sugar for easier use (see below).

This PR builds on top of the "modern layout prototype" from #6531 and is the first step in that direction.

How it works

1. Define "slots":

   {# @Contao/foo.html.twig #}
   <div class="wrapper">
     {# simple usage #}
     {% slot main %}{% endslot %}
   
     {# usage with placeholders #}
     {% slot left %}
         <aside>{{ slot() }}</aside>
     {% endslot %}
     
     {# using optional fallback values #}
     {% slot footer %}
         <aside>{{ slot() }}</aside>
     {% else %}
         <!-- there is no footer -->
     {% endslot %}
   </div>

2. Introspect which slots are available:

   /** @var Contao\CoreBundle\Twig\Inspector\Inspector $inspector */
   $inspector->inspectTemplates('@Contao/foo.html.twig')->getSlots(); // ['footer', 'left', 'main']

3. Set content for slots:

   $twig->render('@Contao/foo.html.twig', [
       '_slots' => [
           'left' => 'content for left slot',
           'main' => 'content for main slot',
       ]
   ]);

4. Expected output:

   <div class="wrapper">
       content for main slot
       <aside>content for left slot</aside>
       <!-- there is no footer -->
   </div>

[Toflar]

Can't say much about whether the parser logic etc. is correct but the way it's used and the syntax is as discussed 😊