Refactor approach to generating .schema.json from .schema.yaml

[nokome]

JSON Schema doesn't have good support for inheritance (i.e. extending/overriding properties defined in other schemas). There are several problems that arise from our current use of the allOf approach:

• it's more complicated than it should be to author schemas that extend others
• well know problem of not being able to specify additionalProperties: false
• can't override the type property with another constants e.g. CreativeWork can't override Thing
• the generated Typescript definitions are suboptimal (require defensive programming) because of the above
• base type and inherited property information is not available for generating docs

The proposed solution is to introduce a $extends keyword and do pre-processing to merge the properties of the base and current types. This would include auto-population of some fields to make schema authoring less tedious and error prone and to add information for docs generation:

• $schema - always http://json-schema.org/draft-07/schema#
• $id - populated with https://stencila.github.com/schema/${schema.title}.schema.json
• properties.type.enum - populated with [${schema.title}]
• properties - merged from the base and current
• properties.*.inherited - populated with title of type that they were inherited from
• required - merged from the base and current and type added
• additionalProperties: false - if not specified

[stencila-ci]

🎉 This issue has been resolved in version 0.9.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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