Blocks field: specify which fields to show when `preview: fields`#623

I love the new ability to quickly enable a preview for custom blocks (link to docs). I think the preview: fields is a great solution and I have a suggestion to make it even better (in my opinion).

To limit the preview complexity and size, Kirby could allow naming sections, tabs or fields in the field.

Examples:

Show all — currently available in K4:

preview: fields

Suggestion: Show a set of specific fields/sections/tabs specified in the block’s blueprint:

preview:
  - fieldName
  - sectionName
  - tabName