Theme block JSON preset completion/hover support

[charlespwd]

Do this one after #625

We want completion of presets.[].(recursive blocks.[]).settings keys & maybe values?

Requirements:

• Key completion of presets.[].blocks.[].settings (same file if local blocks are used, otherwise other file)
• Key completion of presets.[].blocks.[].blocks.[].settings (other file)
• ... and so on :) It's recursive

{% schema %}
{
  "name": "Header Announcement",
  "blocks": [{ "type": "slide" }],
  "settings": [
    {
      "type": "text",
      "id": "my-id",
      "label": "Text"
    }
  ],
  "presets": [
    {
      "name": "Header Announcement",
      "blocks": [
        {
          "type": "slide",
          "settings": {
            "█": "<-- should complete blocks/slide.liquid setting ids"
          },
          "blocks": [
            {
              "type": "group",
              "settings": {
                "█": "<-- should compelte blocks/group.liquid setting ids"
              }
            }
          ]
        }
      ]
    }
  ]
}
{% endschema %}