Update tspd to support named exports for reference docs

https://github.com/microsoft/typespec/pull/4606 added support for named exports of typespec files. However, the `tspd` tool - used for generating decorator typescript types and typespec documentation - currently only works on `package.json#tspMain`. It needs to be updated to work with named exports that contain a `typespec` mapping as well.

### Checklist

- [X] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [X] Read the [docs](https://typespec.io/docs/).
- [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.