zola: Allow shortcodes in shortcode bodies

If a short-code with a body is used in the body of a short-code it’s {% end %} is matched with the outer short-code. This prevents the use of nested short-codes with bodies.

Use Case

I’m trying to write reveal.js presentations in Gutenberg currently and I have defined simple shortcodes for vertical slides, fragments, and notes:

<aside class="notes">
{{ body | safe | trim | markdown }}
</aside>

<section{%- if id %} id="{{id}}" {%- endif -%}
        {%- if attributes -%}
          {%- for attr in attributes %}{#
            #} {{attr.attribute}}="{{attr.value}}"
          {%- endfor -%}
        {%- endif -%}>
{{ body | safe | markdown(inline=true) }}
</section>

<span class="fragment">
    {{ body | safe | trim | markdown }}
</span>

This makes it easier to write slides like so:

+++
title = "Installation instructions"
+++

{% vertical_slide() %}
Main content

{% notes() %}
- notes on main content
{% end %}
{% end %}


{% vertical_slide() %}
Basement slide with extra content

{% notes() %}
- notes on basement content
{% end %}
{% end %}

Workaround

Of course I can just throw in html to work around this, but I find short-codes are more pleasant to work with, especially when I have more complicated things going on like figures with complex markup that I’ve created with short-codes showing up in vertical slides.

For example:

{% vertical_slide() %}
**Linux** (Ubuntu/Debian)

`sudo apt-get install git`

<figure class="toggle-figure">
    <span class="toggle-figure__button"></span>
    <img class="toggle-figure__figure" alt="`sudo apt-get install git`" src="img/gif/linux-install-git.gif"/>
</figure>

<aside class="notes">
- Linux typically comes with `git` installed
- `git` was created specifically to manage the Linux project
</aside>
{% end %}

However, this isn’t ideal since the list inside the aside is not rendered as markdown.