HACK: add default value to class field in format2 schema

This is probably something that should be part of the official schema, but not sure if this is the correct way or should be an enum with a single value. Unless there are more classes supported?

workflow-languages/schemas/gxformat2/v19_09/workflows.yaml

@@ -362,6 +362,8 @@ $graph:
           "_id": "@type"
           "_type": "@vocab"
         type: string
+        # This default is a hack and is not included in the official schema.
+        default: "GalaxyWorkflow"
       - name: steps
         doc: { $include: ../common/steps_description.txt }
         type: