JSON schemas for the manifest versions

[mattias-kindborg-at-work]

The following idea came after reading Manifest schemas.

Many text editors and IDE:s today have either built in, or can be extended with, support for validating JSON files against a JSON schema. Validating the manifest, given its version, before running the actual build would improve the feedback loop, which is nice.

I would be open to contribute the JSON schemas, given that we decide where they should be published. One alternative would be to publish them on JSON Schema Store, another would be to self-host it them one of our repositories. Each alternative has pros and cons.