Warn developers that {% stylesheet %} and {% javascript %} tags only support static content

[karreiro]

Using Liquid code inside {% stylesheet %} or {% javascript %} tags, as shown below, is not permitted:

{% stylesheet %}
  .button {
    background-color: {{ settings.button_color }};
    color: white;
  }
{% endstylesheet %}

This mistake can happen quite easily, so it would be very helpful to have linting in place to catch this scenario.