Give name to IIFE in index.js and expose updateBookCount

Gives a name to the IIFE wrapping code in index.js - kiwixServe
and exposes updateBookCount through it

static/skin/index.js

@@ -1,4 +1,4 @@
-(function() {
+const kiwixServe = (function() {
     const root = document.querySelector(`link[type='root']`).getAttribute('href');
     const incrementalLoadingParams = {
         start: 0,
@@ -493,5 +493,8 @@
         }
         setCookie(filterCookieName, params.toString());
     }
+    return {
+        updateBookCount
+    }
 })();