Replace var with let in some exercise solutions

Issue marijnh#277

Author: marijnh

code/solutions/08_2_the_locked_box.js

@@ -1,5 +1,5 @@
 function withBoxUnlocked(body) {
-  var locked = box.locked;
+  let locked = box.locked;
   if (!locked) {
     return body();
   }

code/solutions/18_3_conways_game_of_life.html

@@ -42,7 +42,7 @@
   // x,y position, counting all fields that have a cell but are not the
   // center field.
   function countNeighbors(grid, x, y) {
-    var count = 0;
+    let count = 0;
     for (let y1 = Math.max(0, y - 1); y1 <= Math.min(height - 1, y + 1); y1++) {
       for (let x1 = Math.max(0, x - 1); x1 <= Math.min(width - 1, x + 1); x1++) {
         if ((x1 != x || y1 != y) && grid[x1 + y1 * width]) {

code/solutions/21_1_disk_persistence.js

@@ -6,7 +6,7 @@ const {readFileSync, writeFile} = require("fs");
 const fileName = "./talks.json";
 
 function loadTalks() {
-  var json;
+  let json;
   try {
     json = JSON.parse(readFileSync(fileName, "utf8"));
   } catch (e) {