Use blocked scoped variables

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

examples/overviewmap.js

@@ -5,7 +5,7 @@ import OverviewMap from '../src/ol/control/OverviewMap.js';
 import TileLayer from '../src/ol/layer/Tile.js';
 import OSM from '../src/ol/source/OSM.js';
 
-var map = new Map({
+const map = new Map({
   controls: defaultControls().extend([
     new OverviewMap()
   ]),