Added the ability to create sub-workspaces in order to enable parent/child (branching) workflows for content

Starting with Drupal 8.8.0, the Workspaces module provides the ability to create sub-workspaces, which automatically inherit all the contents of their parent workspace.

This new functionality enables various use-cases like:

• Entirely new editorial workflows are possible: sub campaign workspaces, regional or country workspaces that inherit content, etc.
• Contributed or custom modules can expose the contents of a workspace via JSON:API or GraphQL, and instantly provide a very competent content-hub solution
• The ability to integrate with (or replace) the Domain Access module, possibly exposing a workspace for each domain

This functionality has been enabled by adding a parent entity reference field to workspace entities, and by tracking each workspace-specific revision in all its descendants.

Given the following workspace hierarchy, here's what's allowed to do with each workspace:

Live
|-- Stage
|   |-- Dev
|   |   |-- Local 1
|   +   \-- Local 2
\-- QA

• Only top-level workspaces can be published to Live (i.e Stage and QA)
• Workspaces that have children can not be deleted (i.e. Stage and Dev)
• Workspace that have a parent are allowed to merge their contents into their parent (i.e. Local 1 -> Dev, Local 2 -> Dev, Dev -> Stage)