bpo-42236: Enhance _locale._get_locale_encoding() (GH-23083)

* Rename _Py_GetLocaleEncoding() to _Py_GetLocaleEncodingObject()
* Add _Py_GetLocaleEncoding() which returns a wchar_t* string to
  share code between _Py_GetLocaleEncodingObject()
  and config_get_locale_encoding().
* _Py_GetLocaleEncodingObject() now decodes nl_langinfo(CODESET)
  from the current locale encoding with surrogateescape,
  rather than using UTF-8.