polyfill: fix wrong calculation of ISO week number

Author: redneb

polyfill/lib/ecmascript.mjs

@@ -2028,7 +2028,7 @@ export const ES = ObjectAssign({}, ES2020, {
     const week = MathFloor((doy - dow + 10) / 7);
 
     if (week < 1) {
-      if (doj === (ES.LeapYear(year) ? 5 : 6)) {
+      if (doj === 5 || (doj === 6 && ES.LeapYear(year - 1))) {
         return 53;
       } else {
         return 52;