gh-108444: Add PyLong_AsInt() public function (#108445)

* Rename _PyLong_AsInt() to PyLong_AsInt().
* Add documentation.
* Add test.
* For now, keep _PyLong_AsInt() as an alias to PyLong_AsInt().

Objects/longobject.c

@@ -549,7 +549,7 @@ PyLong_AsLong(PyObject *obj)
    method.  Return -1 and set an error if overflow occurs. */
 
 int
-_PyLong_AsInt(PyObject *obj)
+PyLong_AsInt(PyObject *obj)
 {
     int overflow;
     long result = PyLong_AsLongAndOverflow(obj, &overflow);