Vendor schemastore `base.json` and inject it into the `referencing` store automatically when loading vendored schemas

[sirosen]

For context:
SchemaStore/schemastore#5381 (comment)

There's an internal-to-schemastore base.json. For offline usage to work, we need that schema to be available in the $ref registry.

1. Vendor it (easy)
2. Detect when a vendored schema is being loaded and store that information for later use (easy)
3. Inject base.json into the registry when the "vendored schema flag" is set (easy?)

It seems simple. 🤞 that it really is!