[Bug] Plugins returning TransformerBasePlugin not supported

[nonara]

ttypescript and earlier tsp supported entry points which returned an object type (see TransformerBasePlugin).

Legacy ttypescript code was labeled improperly and this potential was overlooked after adding wrapper functionality.

This has been corrected, and better error messaging for a bad entry point has been added.

Issue discovered in report:

• https://github.com/nonara/ts-patch/issues/106#issuecomment-1638893611