Support discriminated unions

Is your feature request related to a problem? Please describe.
If I iterate over all the blocks in a section, and each of those blocks have settings. Then if I discriminate on block.type, I should be able to have completion and hover support on block.settings.

{% for block in section.blocks %}
  {% case block.type %}
    {% when 'slide' %}
      {{ block.settings.image }}
    {% else %}
  {% endcase }
  {% if block.type == 'slide' %}
    {{ block.settings.image }}
  {% endif %}
{% endfor %}
{% schema %}
{
  "name": "Slideshow",
  "tag": "section",
  "class": "slideshow",
  "settings": [],
  "blocks": [
    {
      "name": "Slide",
      "type": "slide",
      "settings": [
        {
          "type": "image_picker",
          "id": "image",
          "label": "Image"
        }
      ]
    }
  ]
}
{% endschema %}

Describe the solution you'd like

Do something a bit like typescript where you can discriminate on a union type and get a different one. This is kind of big because it means we might want to support union types as well. e.g. section.blocks being return_type [{ type: block_1 }, { type: block_2 }], etc.

Start with the Liquid example and its schema blocks definition, then trace how section.blocks and block.type are represented for language features. Define the expected completion and hover behavior for block.settings in both case/when and if comparisons, including how multiple block variants are modeled. Done means the example narrows block.settings correctly for the discriminated type.