Use blocked scoped variables

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

examples/popup.js

@@ -10,15 +10,15 @@ import TileJSON from '../src/ol/source/TileJSON.js';
 /**
  * Elements that make up the popup.
  */
-var container = document.getElementById('popup');
-var content = document.getElementById('popup-content');
-var closer = document.getElementById('popup-closer');
+const container = document.getElementById('popup');
+const content = document.getElementById('popup-content');
+const closer = document.getElementById('popup-closer');
 
 
 /**
  * Create an overlay to anchor the popup to the map.
  */
-var overlay = new Overlay({
+const overlay = new Overlay({
   element: container,
   autoPan: true,
   autoPanAnimation: {
@@ -41,7 +41,7 @@ closer.onclick = function() {
 /**
  * Create the map.
  */
-var map = new Map({
+const map = new Map({
   layers: [
     new TileLayer({
       source: new TileJSON({
@@ -63,8 +63,8 @@ var map = new Map({
  * Add a click handler to the map to render the popup.
  */
 map.on('singleclick', function(evt) {
-  var coordinate = evt.coordinate;
-  var hdms = _ol_coordinate_.toStringHDMS(toLonLat(coordinate));
+  const coordinate = evt.coordinate;
+  const hdms = _ol_coordinate_.toStringHDMS(toLonLat(coordinate));
 
   content.innerHTML = '<p>You clicked here:</p><code>' + hdms +
       '</code>';