Upgrade schema to JSON Schema v2020-12

[jugaadi]

Using this tool, the existing Draft 7 schema can be converted to v2020-12. The new version has also been adopted as the official version in OpenAPI v3.1.

However, the tooling is yet to catch up with the new version and therefore, I would suggest that we maintain a v2020-12 directory inside the Vega directory for specifying these schemas.

[domoritz]

We use https://github.com/vega/ts-json-schema-generator to generate the schema (at least for Vega-Lite) so I think it could make sense to update that when tooling (like vscode and Ajv)are updated.

[jugaadi]

Both VSCode and AJV supports v2020-12.

[domoritz]

Then should we update the generator?