libc: Add WCHAR_WIDTH in <wchar.h> as well

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

include/wchar.h

@@ -97,6 +97,9 @@ typedef	__wint_t	wint_t;
 
 #define	WCHAR_MIN	__WCHAR_MIN
 #define	WCHAR_MAX	__WCHAR_MAX
+#if __ISO_C_VISIBLE >= 2023
+#define	WCHAR_WIDTH	__WCHAR_WIDTH
+#endif
 
 #ifndef WEOF
 #define	WEOF 	((wint_t)-1)