(Stretch) Update the theme-tools linter to present a warning when the header has unused parameters

- [x] Create a rule to check if a doc variable is used in the file
- [x] Ensure that rule does not have false-negatives where the variable is scoped to a loop


```
{% doc %}
  @param param1 - example param
  @param param2 - example param
{% enddoc %}

{{ param1 }} // Good

{% for param2 in array %}
  {{ param2 }} // This isn't using the param2 passed into the snippet
{% endfor %}
```