Reintroduce down level DTS package into Core SDK

With the creation of the `lib-extensions` package in the Core SDK package, we need to add back support for the down-level DTS package in the Core SDK. This is specifically to generate typescript 3.8 types (the older typescript version OCP console uses) for the core extensions we will be adopting.

AC

• Core SDK lib-extensions package produces typescript 3.8 types for extensions that OCP console will adopt