Check that authored YAML conforms to a schema before processing

[ddbeck]

Declaring a schema for authored files (with internal-only fields like draft_date and compute_from) and validating files against it would prevent certain kinds of mistakes, like setting compute_from instead of status.compute_from. See https://github.com/web-platform-dx/web-features/pull/1703/files#r1741081696 for an example of the kind of error I would like to catch.

I wonder if using a type validation library, like zod, would help here. We do some ad-hoc validation, but nothing comprehensive.