Use blocked scoped variables

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

test/spec/ol/format/xsd.test.js

@@ -5,7 +5,7 @@ describe('ol.format.XSD', function() {
 
   describe('readDateTime', function() {
     it('can handle non-Zulu time zones', function() {
-      var node = document.createElement('time');
+      const node = document.createElement('time');
       node.textContent = '2016-07-12T15:00:00+03:00';
       expect(new Date(XSD.readDateTime(node) * 1000).toISOString()).to.eql('2016-07-12T12:00:00.000Z');
     });