Upgrade @hyperjump/json-schema

[somewhatabstract]

@hyperjump/json-schema requires a minimum of Node 18 - we're moving to Node 20 with #1714. In addition, we will need to either migrate to something like vitest or update Jest to use the experimental ESM support as the updated package is ESM-only.

[somewhatabstract]

This is not so simple. Latest @hyperjump/json-schema is ESM-only so we'll have to move to support that, which requires some work to get things working in jest properly. We can avoid having to use ESM file extensions if we configure things like TypeScript appropriately.