bpo-46008: Move Py*State init into distinct functions. (gh-29977)

bpo-46008: Move Py*State init into distinct functions. (gh-29977)

Previously, basic initialization of PyInterprterState happened in PyInterpreterState_New() (along with allocation and adding the new interpreter to the runtime state). This prevented us from initializing interpreter states that were allocated separately (e.g. statically or in a free list). We've addressed that here by factoring out a separate function just for initialization. We've done the same for PyThreadState. _PyRuntimeState was sorted out when we added it since _PyRuntime is statically allocated. However, here we update the existing init code to line up with the functions for PyInterpreterState and PyThreadState.

https://bugs.python.org/issue46008