gh-139353: Rename formatter_unicode.c to unicode_formatter.c (#139723)

* Move Python/formatter_unicode.c to Objects/unicode_formatter.c.
* Move Objects/stringlib/localeutil.h content into
  unicode_formatter.c. Remove localeutil.h.
* Move _PyUnicode_InsertThousandsGrouping() to unicode_formatter.c
  and mark the function as static.
* Rename unicode_fill() to _PyUnicode_Fill() and export it in
  pycore_unicodeobject.h.
* Move MAX_UNICODE to pycore_unicodeobject.h as _Py_MAX_UNICODE.