Implement the snippet tag

[jg-rp]

This PR implements the {% snippet %} tag, new in Shopify/liquid 5.10.0.

TODO:

• Subclass BoundTemplate for inline instances so we get a reasonable string representation when output.

Inline snippets

A snippet is a reusable block of Liquid markup. Traditionally we'd save a snippet to a file and include it in a template with the {% render %} tag.

{% render  "some_snippet" %}

Now, with the {% snippet %} tag, we can define blocks for reuse inside a single template, potentially reducing the number of snippet files we need.

{% snippet div %}
  <div>
    {{ content }}
  </div>
{% endsnippet %}

Defining a snippet does not render it. We use {% render snippet_name %} to render a snippet, where snippet_name is the name of your snippet without quotes (file-based snippet names must be quoted).

{% snippet div %}
  <div>
    {{ content }}
  </div>
{% endsnippet %}

{% render div, content: "Some content" %}
{% render div, content: "Other content" %}

Output

  <div>
    Some content
  </div>


  <div>
    Other content
  </div>

Inline snippets share the same namespace as variables defined with {% assign %} and {% capture %}, so be wary of accidentally overwriting snippets with non-snippet data.

{% snippet foo %}Hello{% endsnippet %}
{% foo = 42 %}
{% render foo %} {% # error %}

Snippets can be nested and follow the same scoping rules as file-based snippets.

{% snippet a %}
  b
  {% snippet c %}
    d
  {% endsnippet %}
  {% render c %}
{% endsnippet %}

{% render a %}
{% render c %} {% # error, c is out of scope %}

Snippet blocks are bound to their names late. You can conditionally define multiple snippets with the same name and pick one at render time.

{% if x %}
  {% snippet a %}b{% endsnippet %}
{% else %}
  {% snippet a %}c{% endsnippet %}
{% endif %}
{% render a %}