Checklist

• I'm reporting a feature request
• I've checked for similar feature requests including closed ones

Description

We maintain quite a few huge JSON files in this repository.
It's not impossible that human-error does it's thing and we have typos in fields, or property values that don't make sense, or accidentally add redundant properties etc.

I think we should make a JSON Schema that validates the contents of the JSON files, and we can define them at $schema.

This will:

1. Add code completion and validation on IDEs and code editors when contributors modify the file, vastly improving the contribution experience.
2. Allow us to strictly define and document all rules / properties that can be set in the JSON files.
3. Maybe later we can add a validation step in CI to ensure the schema matches. (Maybe this could be in code as well for when users use a custom JSON file via the --json argument.)