Issue #9542: Create PyUnicode_FSDecoder() function

It's a ParseTuple converter: decode bytes objects to unicode using
PyUnicode_DecodeFSDefaultAndSize(); str objects are output as-is.

 * Don't specify surrogateescape error handler in the comments nor the
   documentation, but PyUnicode_DecodeFSDefaultAndSize() and
   PyUnicode_EncodeFSDefault() because these functions use strict error handler
   for the mbcs encoding (on Windows).
 * Remove PyUnicode_FSConverter() comment in unicodeobject.c to avoid
   inconsistency with unicodeobject.h.