gh-76785: Move the Cross-Interpreter Code to Its Own File (gh-111502)

This is partly to clear this stuff out of pystate.c, but also in preparation for moving some code out of _xxsubinterpretersmodule.c.  This change also moves this stuff to the internal API (new: Include/internal/pycore_crossinterp.h).  @vstinner did this previously and I undid it.  Now I'm re-doing it. :/