Use blocked scoped variables

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

examples/arcgis-image.js

@@ -5,10 +5,10 @@ import ImageLayer from '../src/ol/layer/Image.js';
 import OSM from '../src/ol/source/OSM.js';
 import ImageArcGISRest from '../src/ol/source/ImageArcGISRest.js';
 
-var url = 'https://sampleserver1.arcgisonline.com/ArcGIS/rest/services/' +
+const url = 'https://sampleserver1.arcgisonline.com/ArcGIS/rest/services/' +
     'Specialty/ESRI_StateCityHighway_USA/MapServer';
 
-var layers = [
+const layers = [
   new TileLayer({
     source: new OSM()
   }),
@@ -20,7 +20,7 @@ var layers = [
     })
   })
 ];
-var map = new Map({
+const map = new Map({
   layers: layers,
   target: 'map',
   view: new View({