Add a low-level interface for writing GitHub actions in Organist

[thufschmitt]

Create a small layer on top of the files.* api (#58) to generate a GitHub actions workflow, with a contract ensuring a minimal amount of validity checking and autocompletion.

The contract might be generated with https://github.com/nickel-lang/json-schema-to-nickel and https://json.schemastore.org/github-workflow.json or handwritten (or a mix of both) depending on which method gives the best and quickest results