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">
  {{ foo | append: '.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.

[bakura10]

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

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

[karreiro]

{{ 'loading-spinner-{{ foo }}.svg' |
Is this a typo? Since when such an awesome syntax is possible ? 😍

Oops! I just fixed that. That syntax would be interesting indeed, but it's not part of this proposal 😅

Thank you for catching that, @bakura10!

[gmalette]

When we change the syntax of a programming language, we have to carefully consider which previously invalid programs we're making valid, and conversely which valid programs we're making invalid (aka breaking the sytnax). Making valid programs invalid is undesirable, and thus we should always strive to always only allow the smallest set of programs as valid; we always have the options to make them valid in the future.

I think we should take the opportunity to refine the spec of the doc tag more, and only allow what we actually want. This will make adding features to it much easier. Making sure that we can't parse invalid templates will prevent them from being saved, and means we won't accrue unexpected usages that become a nightmare to support in the future (hello {% comment %}).

Right now, the syntax allows for almost anything within the block body.

Valid Programs

Otherwise invalid tags

{% doc %}
  {% if %}
  {% comment %}
  {% %}
{% enddoc %}

Any attributes

{% doc %}
  @params foo
  @paramssss foo
  @potato
  @param
  @example
{% enddoc %}

Repeated attributes

{% doc %}
  @param bar
  @param bar
  @param {Product} product
  @param {ProductVariant} product
{% enddoc %}

Unspecified sytnax attribute

{% doc %}
  @param [string} this is the bar attribute | `bar`
{% enddoc %}

Attribute ordering

{% doc %}
  Just a comment

  @params foo - this is foo

  Is this just a comment or part of params?

  @example 
  {% render 'snippet', foo: true %}

  Also this, what is this?

  @param bar - this is bar

  @example 
  {% render 'snippet', bar: true %}
{% enddoc %}

Invalid Programs

AFAICT from the doc, these are the only invalid brands of doc programs:

Unclosed moustache

This invalid by virtue of forcing unclosed doc

{% doc %}
  {{
{% enddoc %}

Unclosed tag

This invalid by virtue of forcing unclosed doc

{% doc %}
  {% 
{% enddoc %}

Nested doc

Intentionally disallowed

{% doc %}
  {% doc %}
{% enddoc %}

[gmalette]

Note, I'm not formulating an opinion on whether any of the currently valid programs should remain or should change. What I want is for the spec (and implementation) to make this determination

[ggmichaelgo]

@GuillermoShopify I believe the doc tag should parse the tag body like the raw tag and allow incomplete Liquid codes like this:

{% doc %}
  {% assign x = ...
{% enddoc %}

With the comment tag we can't support this syntax due to backward compatibility, but we shouldn't make that a case for the new doc tag.