Update tspd to support named exports

[chrisradek]

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
• [X] Read the docs.
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.