gh-141070: Add PyUnstable_Object_Dump() function (#141072)

* Promote _PyObject_Dump() as a public function.
* Keep _PyObject_Dump() alias to PyUnstable_Object_Dump()
  for backward compatibility.
* Replace _PyObject_Dump() with PyUnstable_Object_Dump().

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Kumar Aditya <kumaraditya@python.org>
Co-authored-by: Petr Viktorin <encukou@gmail.com>