Enhancement: Better support for using block fields in block fields

[estruyf]

Currently, it is not possible to create the following structure:

ingredients:
  - wet:
    - water:
    	quantity: 1
    	unit: cup
  - dry:
    - flour:
    	quantity: 10
    	unit: oz

[estruyf]

In order to support this, changes have been made to the block renderer.

The field configuration looks as follows:

{
  "frontMatter.taxonomy.contentTypes": [
    {
      "name": "Recipe",
      "pageBundle": false,
      "previewPath": null,
      "fields": [
        {
          "title": "Ingredients",
          "name": "ingredients",
          "type": "block",
          "fieldGroup": ["wet", "dry"]
        }
      ]
    }
  ],
  "frontMatter.taxonomy.fieldGroups": [
    {
      "id": "wet",
      "fields": [
        {
          "title": "Wet",
          "name": "wet",
          "type": "block",
          "fieldGroup": ["water", "oil"]
        }
      ]
    },
    {
      "id": "water",
      "fields": [
        {
          "title": "Water",
          "name": "water",
          "type": "fields",
          "fields": [
            {
              "title": "Quantity",
              "name": "quantity",
              "type": "string",
              "single": true
            },
            {
              "title": "Unit",
              "name": "unit",
              "type": "string",
              "single": true
            }
          ]
        }
      ]
    },
    ...
  ]
}

Output looks as follows in front matter:

ingredients:
  - wet:
      - water:
          quantity: "123"
          unit: Cups
        fieldGroup: water
    fieldGroup: wet