Visual Editing backend supports moving a compound

To provide drag-and-drop functionality on the frontend, we should add another endpoint for updating the order of a compound value:

PATCH documents/{documentId}/editable/{fieldPath}

The input is similar to the parameters accepted by adding new values:

For example, if the second value needs to become fourth, the call will look something like:

PATCH documents/uuid/editable/hap:employee[2]