gh-93103: Py_DecodeLocale() uses _PyRuntime.preconfig (#93187)

The Py_DecodeLocale() and Py_EncodeLocale() now use
_PyRuntime.preconfig, rather than Py_UTF8Mode and
Py_LegacyWindowsFSEncodingFlag global configuration varibles, to
decide if the UTF-8 encoding is used or not.

As documented, these functions must not be called before Python is
preinitialized. The new PyConfig API should now be used, rather than
using deprecated functions like Py_SetPath() or PySys_SetArgv().