[Bug]: Discriminated Types not supported by JSON emitter

### Describe the bug

[Discriminated Types](https://typespec.io/docs/standard-library/discriminated-types/) feature is not supported in JSON schemas. The exported schemas do not contain the discriminator. See the playground link which uses example from the documentation.

### Reproduction

https://typespec.io/playground/?c=aW1wb3J0ICJAdHlwZXNwZWMvanNvbi1zY2hlbWEiOwoKdXNpbmcgSnNvblPFFDsKCkDEI8YOCm5hbWVzcGFjZSDGEXPEIGRpc2NyaW1pbmF0ZWQKdW5pb24gUGV0IHsKICBjYXQ6IENhdCwKICBkb2c6IERvZywKfQoKbW9kZWzEGsUnxFg6IHN0cmluZzsKICBtZW93OiBpbnQzMjvKLkRvZ9UuYmFya8oQfQ%3D%3D&e=%40typespec%2Fjson-schema&options=%7B%7D&vs=%7B%7D

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/Microsoft/typespec/discussions).
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.