To build on the previous work supporting querying Workflows for GitHub
Actions, add a class and relevant methods to support querying runs of a
workflow.
When Workflow support was added, the method calls were not added to
the typing file for Repository, and somehow the class name for the
Workflow typing file was incorrect.
To start supporting GitHub Actions, add the first part of that,
Workflow, which encapsulates a workflow over the API, along with
two methods on Repository to query them.