BasicComponents should support "if blocks.foo" statements

[nerdoc]

Many times, in components you need to render some tags depending on a block being present or not, especially in "easy" BasicComponents:

E.g. In a Bootstrap card, if there is a title, I want to render a card-header > card-title div, and inside, the block with the title:

django-web-components do that like this:

{% if slots.title or slots.actions %}
<div class="card-header">
        {% if slots.title %}
        <h3 class="card-title">
            {% render_slot slots.title %}
        </h3>
        {% endif %}

        {% if slots.actions %}
        <div class="card-actions">
            {% render_slot slots.actions %}
        </div>
        {% endif %}
</div>
{% endif %}

Tetra can't do that. But Tetra components could support a blocks (or block) variable that just has booleans as members, named after the available blocks. So you could use {% if blocks.title %} <div class="foo">{% block title %}</div>{% endif %}

So the wrapper div is not rendered if the block is not present.

This would ease creating components a lot.

[nerdoc]

Done in Tetra 0.1.0