Add a JSON schema and validate against it

[frankieroberto]

Currently we have a check which makes sure the JSON files are valid JSON and tidied, but no schema. This means that typos can creep in. There’s also no consistent order to the keys in the JSON files.

Ideally there would be some kind of schema, and then a check would run against it when opening PRs. The keys could be automatically re-ordered to be consistent too.