Support YAML anchors

---
version: 1.0
description: Deploy staging application
.definitions:
  action_succeeded: &action_succeeded
    - task_message: "<% ctx('environment') %> deploy <% task().task_name %> :white_check_mark:"
 
  someaction:
    action: core.noop
    retry:
      count: 3
      delay: 60
    next:
      - when: <% succeeded() %>
        publish: *action_succeeded
        do:
          - notify_slack
      - when: <% failed() %>
        publish:
          - task_message: "<% ctx('environment') %> deploy <% task().task_name %> :boom:"
          - stderr: <% result().stderr %>
        do:
          - notify_slack
          - fail
...

However when I run the workflow I get:

{
  "errors": [
    {
      "message": "Additional properties are not allowed ('.definitions' was unexpected)",
      "spec_path": null,
      "schema_path": "additionalProperties",
      "type": "syntax"
    }
  ],
  "output": null
}

Start with the example workflow and the reported validation error, then trace the workflow YAML validation path that rejects the .definitions key and YAML anchor reference. Done means the shown workflow is accepted and the anchored action_succeeded values are usable when the workflow runs, without the unexpected-property error.