Add support to LiquidDoc with the new {% doc %} tag

[karreiro]

This PR implements RFC#1865 and introduces support for the new {% doc %} tag, which will be used to document elements of Liquid templates, primarily snippets.

Snippets are the primary method for reusing logic in Liquid. However, their weak interfaces make it easy to overlook dependencies when rendering them.

By adopting the emerging pattern of using headers on these snippets, the {% doc %} tag will document those interfaces. This way, tools such as the Liquid language server can support features around these definitions, such as code completion, better linting rules, and inline documentation. Here's an example of {% doc %} in action:

{% doc %}
  Renders loading-spinner.

  @param {string} foo - some foo
  @param {string} [bar] - optional bar

  @example
  {% render 'loading-spinner', foo: 'foo' %}
  {% render 'loading-spinner', foo: 'foo', bar: 'bar' %}
{% enddoc %}

<div class="{{ bar }} loading__spinner hidden">
  {{ 'loading-spinner-{{ foo }}.svg' | inline_asset_content }}
</div>

This new tag should behave just like the {% comment %} tag and have no effect on rendering. Therefore, to implement this, the Doc tag inherits from Comment, similar to how Unless inherits from If.

[charlespwd]

One of the weird things about comment is that you can nest them. Do we really want similar behaviour for doc? I don't think we do 🤔?

[bakura10]

{{ 'loading-spinner-{{ foo }}.svg' |

Is this a typo? Since when such an awesome syntax is possible ? 😍