field.new-schema.yml

field_instance.text_textarea_with_summary.settings:
  type: mapping
  label: 'Text area with a summary'
  mapping:
    text_processing:
      type: string
      label: 'Text processing'
    display_summary:
      type: boolean
      label: 'Summary input'
    user_register_form:
      type: boolean
      label: 'Display on user registration form.'

field_instance.text_textarea_with_summary.default:
  type: sequence
  label: 'Defaule value'
  sequence:
    - type: mapping
      label: 'Default'
      mapping:
        summary:
          type: string
          label: 'Summary'
        value:
          type: text
          label: 'Body'
        format:
          type: string
          label: 'Text format'


# Tried to combine above two data types to single type like below:
field_instance.text_textarea_with_summary.*
  type: mapping
  label: 'Text area with a summary'
  mapping:
    settings:
      type: mapping
      label: 'Text area with a summary'
      mapping:
        text_processing:
          type: string
          label: 'Text processing'
        display_summary:
          type: boolean
          label: 'Summary input'
        user_register_form:
          type: boolean
          label: 'Display on user registration form.'
    default:
      type: sequence
      label: 'Defaule value'
      sequence:
        - type: mapping
          label: 'Default'
          mapping:
            summary:
              type: string
              label: 'Summary'
            value:
              type: text
              label: 'Body'
            format:
              type: string
              label: 'Text format'

# and it didn't work :(