gh-101758: Add _PyState_AddModule() Back for the Stable ABI (gh-101956)

We're adding the function back, only for the stable ABI symbol and not as any form of API. I had removed it yesterday.

This undocumented "private" function was added with the implementation for PEP 3121 (3.0, 2007) for internal use and later moved out of the limited API (3.6, 2016) and then into the internal API (3.9, 2019). I removed it completely yesterday, including from the stable ABI manifest (where it was added because the symbol happened to be exported). It's unlikely that anyone is using _PyState_AddModule(), especially any stable ABI extensions built against 3.2-3.5, but we're playing it safe.

https://github.com/python/cpython/issues/101758