[ilan-gold]

Towards #74, this gives a schema for the "standard" formats. Two things:

1. I wasn't exactly clear where the custom ones go or how they interact with the other keys or where the custom-keyed schema goes. It might make sense to make it a separate schema but not sure?
2. This is probably out-of-scope if additional keys are disallowed given the lack of need to otherwise bump a version, but the json objects themselves can declare which schema they adhere to via $schema which is more of a convention but still could be useful at some point.
3. Does this belong in drafts as well? I am not totally clear how a "build" would work with the schema, which I think should be for public consumption by other readers/writers

Sorry if you had already started this @willow-ahrens but I didn't see any branches here or the like and you don't seem to have a fork.