gh-119182: Use public PyUnicodeWriter in ast_unparse.c (#129208)

Replace the private _PyUnicodeWriter API with the public
PyUnicodeWriter API.

* Add append_char() function.
* Add APPEND_CHAR() and APPEND_CHAR_FINISH() macros.
* Replace APPEND_STR() and APPEND_STR_FINISH() of single character
  with APPEND_CHAR() and APPEND_CHAR_FINISH().

Author: vstinner