Replace a bunch of Array.prototype.forEach() cases with for...of loops instead

Using `for...of` is a modern and generally much nicer pattern, since it gets rid of unnecessary callback-functions. (In a couple of spots, a "regular" `for` loop had to be used.)

src/core/evaluator.js

@@ -1394,9 +1394,9 @@ class PartialEvaluator {
       ) {
         const groupIds = [];
         if (Array.isArray(optionalContentGroups)) {
-          optionalContent.get("OCGs").forEach(ocg => {
+          for (const ocg of optionalContentGroups) {
             groupIds.push(ocg.toString());
-          });
+          }
         } else {
           // Dictionary, just use the obj id.
           groupIds.push(optionalContentGroups.objId);