Fix file extensions for chapter 21 solutions

Author: marijnh

code/solutions/06_3_iterable_groups.js

@@ -30,17 +30,17 @@ class Group {
 
 class GroupIterator {
   constructor(members) {
-    this.members = members;
-    this.position = 0;
+    this.#members = members;
+    this.#position = 0;
   }
 
   next() {
-    if (this.position >= this.members.length) {
+    if (this.#position >= this.#members.length) {
       return {done: true};
     } else {
-      let result = {value: this.members[this.position],
+      let result = {value: this.#members[this.#position],
                     done: false};
-      this.position++;
+      this.#position++;
       return result;
     }
   }

code/solutions/21_1_disk_persistence.js renamed to code/solutions/21_1_disk_persistence.mjs

@@ -99,18 +99,18 @@ for (let file of fs.readdirSync(".").sort()) {
   ];
   if (chapter.number == 21) chapter.exercises = [
     {name: "Disk persistence",
-     file: "code/solutions/21_1_disk_persistence.js",
+     file: "code/solutions/21_1_disk_persistence.mjs",
      number: 1,
      type: "js",
      code: nodeInfo,
-     solution: fs.readFileSync("code/solutions/21_1_disk_persistence.js", "utf8")
+     solution: fs.readFileSync("code/solutions/21_1_disk_persistence.mjs", "utf8")
     },
     {name: "Comment field resets",
-     file: "code/solutions/21_2_comment_field_resets.js",
+     file: "code/solutions/21_2_comment_field_resets.mjs",
      number: 2,
      type: "js",
      code: nodeInfo,
-     solution: fs.readFileSync("code/solutions/21_2_comment_field_resets.js", "utf8")
+     solution: fs.readFileSync("code/solutions/21_2_comment_field_resets.mjs", "utf8")
     }
   ];