[sections] missing types of nested elements

[kptdobe]

The mdast generated for a section with only one image looks like this:

{
  type: 'paragraph,
  children: [{
    type: 'image'
  }]
}

The resulting types contain only has-paragraph because the logic does not iterate through the children.
More generally, all types of elements are encapsulated inside a parent paragraph, the types property does not compute meaningful info for most of the use cases.