New UnusedDocParam theme-check rule

[aswamy]

What are you adding in this PR?

Closes #803

• Add check to ensure @param variable is actually used in the snippet

Questions

Why didn't i update UnusedAssign as well to ignore loop-scoped variables

UnusedAssign can give false-positive if we also ignore loop-scoped variables because of situations like this:

{% assign foo = 'example' %} // Would have UnusedAssign variable: GOOD

{% for foo in array %}
  {% assign foo = 'override' %} // Would have UnusedAssign variable: BAD
  {{ foo }}
{% endfor %}

• We would have to maintain a list of all variables and their scopes, which wouldn't have a good effort/value ratio

Tophat

• Have a snippet with doc
• Ensure the params defined are actually used
• We will consider it "unused" if the variable is actually a for-loop scoped variable
• You can hover over the warning, and press quick fix to remove the param

e.g.

{% doc %}
  @param {string} name - This is a name.
  @param {object} opts - options // Unused
  @param {unknown} extra - extra data // Unused
{% enddoc %}

{{ name }}

{% tablerow extra in product.variants %}
  {{ extra }}
{% endtablerow %}

{% for extra in product.variants %}
  {{ extra }}
{% endfor %}

Before you deploy

• I included a minor bump changeset
• My feature is backward compatible

[jamesmengo]

Left a couple of nits but functionally ✅