Support string iterator for strings longer than 64K (#4727)

JerryScript-DCO-1.0-Signed-off-by: Zoltan Herczeg zherczeg.u-szeged@partner.samsung.com

tests/jerry/es.next/string-iterator.js

@@ -75,3 +75,15 @@ assert(next.value === undefined);
 
 assert(iterator.toString() === '[object String Iterator]');
 assert(iterator[Symbol.toStringTag] === 'String Iterator');
+
+iterator = (' '.repeat(80000))[Symbol.iterator]();
+next = iterator.next();
+idx = 0;
+
+while (!next.done) {
+  assert(next.value === ' ');
+  assert(next.done === false);
+  next = iterator.next();
+  idx++;
+}
+assert(idx == 80000);