Add `schema_generator` for generating JSON schemas (#23991)

This PR adds a `schema_generator` crate that can be used to generate our
various JSON schemas for publishing elsewhere.

Currently it does the simplest thing possible and just prints the JSON
schema to stdout. We can make this a but more robust later.

I also removed the schema-printing facilities from the `theme_importer`,
as they don't really make sense there.

Release Notes:

- N/A