Build and maintain an official JSON schema for pubspec.yaml

[jonasfj]

There is a JSON schema here:
https://github.com/SchemaStore/schemastore/blob/master/src/schemas/json/pubspec.json

However, it has some bugs, for example on publish_to: none, because it uses oneOf instead of using anyOf.

I think we should fix that. Moreover, I think we should declare and maintain an official JSON schema in this repository.

Possible, we should also have test cases and such.
We can settle with only running test cases in CI, using ajv (npm + node.js). I'm thinking we'd benefit from writing the schema in YAML, and would need to export it as JSON for others.

We could also work to become code owners in SchemaStore (if they'll have us 🤣).

We could also use JSON schema to generate: https://dart.dev/tools/pub/pubspec
This is just a possibly crazy idea. There are also tools like docson to render JSON schemas as documentation. Something worth considering, hopefully there are newer and better tools too.

This could be a good contribution opportunity.

Full disclosure: I haven't thought all of this through. Could be reasons we don't want to do this.