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/display/api.js

@@ -2289,11 +2289,11 @@ class WorkerTransport {
     const waitOn = [];
     // We need to wait for all renderings to be completed, e.g.
     // timeout/rAF can take a long time.
-    this.pageCache.forEach(function (page) {
+    for (const page of this.pageCache) {
       if (page) {
         waitOn.push(page._destroy());
       }
-    });
+    }
     this.pageCache.length = 0;
     this.pagePromises.length = 0;
     // Allow `AnnotationStorage`-related clean-up when destroying the document.