Enable the ESLint no-var rule in the examples/ folder

Updating the examples to use `let`/`const` should be fine, given that they are available in all browsers/platforms that the PDF.js library now supports; please note the following compatibility information:

 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#browser_compatibility
 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const#browser_compatibility

examples/.eslintrc

@@ -8,4 +8,9 @@
     "pdfjsLib": false,
     "pdfjsViewer": false,
   },
+
+  "rules": {
+    // ECMAScript 6
+    "no-var": "error",
+  },
 }

examples/mobile-viewer/viewer.js

@@ -35,7 +35,7 @@ const MIN_SCALE = 0.25;
 const MAX_SCALE = 10.0;
 const DEFAULT_SCALE_VALUE = "auto";
 
-var PDFViewerApplication = {
+const PDFViewerApplication = {
   pdfLoadingTask: null,
   pdfDocument: null,
   pdfViewer: null,
@@ -425,6 +425,8 @@ var PDFViewerApplication = {
   },
 };
 
+window.PDFViewerApplication = PDFViewerApplication;
+
 document.addEventListener(
   "DOMContentLoaded",
   function () {

examples/node/domstubs.js

@@ -188,9 +188,9 @@ DOMElementSerializer.prototype = {
         this._loopIndex = 0;
       /* falls through */
       case 5: // Serialize child nodes (only for non-tspan/style elements).
-        var value;
         while (true) {
-          value = this._childSerializer && this._childSerializer.getNext();
+          const value =
+            this._childSerializer && this._childSerializer.getNext();
           if (value !== null) {
             return value;
           }