Define a YAML schema if possible

[StevenMaude]

It feels like this repository is the best place to open this issue, as actions are checked here. But feel free to close and tell me where to open it if more appropriate.

A user asked a question, that I think should be resolved by fixing the indentation of their project.yaml. But opensafely doesn't complain about the configuration; it is valid YAML and the describe top-level key in that example is just silently ignored.

If we had schema validation, then we could add this to the Gitpod configuration for the research template for Gitpod. This might alert users if something is wrong with the configuration.

[ghickman]

I think we might be able to generate a schema now that pydantic is validating the configuration, or at the very least use this library as a validator.