Add missing schema of field formatter "cshs_group_by_root"

Problem/Motivation

The cshs.schema.yml is missing the definition of the CSHS Group By Root (cshs_group_by_root) field format.

Proposed resolution

Add the following configuration in the yaml file.

field.formatter.settings.cshs_group_by_root:
  type: mapping
  label: 'CSHS Group By Root settings'
  mapping:
    linked:
      type: boolean
      label: 'Linked'
    reverse:
      type: boolean
      label: 'Reverse'
    sort:
      type: string
      label: 'Sort'
    depth:
      type: integer
      label: 'Depth'
    last_child:
      type: boolean
      label: 'Last child'