Add nullish coalescing to multiple articles, refactor operators, renumber the chapter

1-js/02-first-steps/13-while-for/5-replace-for-while/task.md

@@ -0,0 +1,14 @@
+importance: 5
+
+---
+
+# Replace "for" with "while"
+
+Rewrite the code changing the `for` loop to `while` without altering its behavior (the output should stay same).
+
+```js run
+for (let i = 0; i < 3; i++) {
+  alert( `number ${i}!` );
+}
+```
+