Implement DateDiff(interval, date1, date2 [, firstdayofweek [, firstweekofyear]]) with support for all interval types: yyyy, q, m, y, d, w, ww, h, n, s. For yyyy/q/m the difference is computed from calendar fields via VarUdateFromDate. For d/y the day number difference is used. For w it is the day difference integer-divided by 7 (not affected by firstdayofweek). For ww it counts week boundary crossings based on firstdayofweek. For h/n/s the dates are converted to the respective unit and truncated before subtracting. Returns VT_I4 (Long). Null date arguments return Null; null interval or null firstdayofweek/firstweekofyear return VBSE_ILLEGAL_NULL_USE. -- v8: vbscript: Implement DateDiff built-in function. https://gitlab.winehq.org/wine/wine/-/merge_requests/10459