Use blocked scoped variables

In addition to using const and let, this also upgrades our linter config and removes lint (mostly whitespace).

examples/zoomslider.js

@@ -11,14 +11,12 @@ import OSM from '../src/ol/source/OSM.js';
  * @param {string} divId The id of the div for the map.
  * @return {ol.PluggableMap} The ol.Map instance.
  */
-var createMap = function(divId) {
-  var source, layer, map, zoomslider;
-
-  source = new OSM();
-  layer = new TileLayer({
+function createMap(divId) {
+  const source = new OSM();
+  const layer = new TileLayer({
     source: source
   });
-  map = new Map({
+  const map = new Map({
     layers: [layer],
     target: divId,
     view: new View({
@@ -26,11 +24,11 @@ var createMap = function(divId) {
       zoom: 2
     })
   });
-  zoomslider = new ZoomSlider();
+  const zoomslider = new ZoomSlider();
   map.addControl(zoomslider);
   return map;
-};
+}
 
-var map1 = createMap('map1');
-var map2 = createMap('map2');
-var map3 = createMap('map3');
+const map1 = createMap('map1');
+const map2 = createMap('map2');
+const map3 = createMap('map3');