Simplified definition format for sequences in configuration schemas

Drupal 8 configuration schemas include the possibility to define a list of items which have the same type or at least resolve from the same dynamic type definition. This type is called a sequence. While all of configuration schema are key/value based, type definitions for sequences required a one item list in the definition itself. This was a syntax gimmick and not necessary. Therefore the list level is now removed and all keys should be specified the same way as for any other type (especially mappings).

Before:

views_field_bulk_form:
  type: views_field
  label: 'Bulk operation'
  mapping:
    [...]
    selected_actions:
      type: sequence
      label: 'Available actions'
      sequence:
        - type: string     # Note the hyphen in this example.
          label: 'Action'  # Additional details were organized under that hyphen.

theme_settings:
  type: mapping
  mapping:
    [...]
    third_party_settings:
      type: sequence
      label: 'Third party settings'
      sequence:
        - type: theme_settings.third_party.[%key]   # Note the hyphen in this example.

AFTER:

views_field_bulk_form:
  type: views_field
  label: 'Bulk operation'
  mapping:
    [...]
    selected_actions:
      type: sequence
      label: 'Available actions'
      sequence:
        type: string     # Note no use of hyphen anymore.
        label: 'Action'  # Further details are on the same level.

theme_settings:
  type: mapping
  mapping:
    [...]
    third_party_settings:
      type: sequence
      label: 'Third party settings'
      sequence:
        type: theme_settings.third_party.[%key]  # Note no hyphen anymore.

Once again, the difference is that the sequence item type is not defined anymore in a one item list but with additional keys under the sequence definition.

The old list format is still supported but only for backwards compatibility and will be removed in Drupal 11.