Define __WCHAR_WIDTH in sys/_types.h and derive WCHAR_WIDTH from
that, the same way as WCHAR_MIN and WCHAR_MAX, in both <wchar.h>
and <stdint.h> as per C23 §7.31.1 and §7.22.3.4, respectively.
Reviewed by: fuz
Approved by: fuz (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D59385