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

1-js/02-first-steps/13-while-for/4-for-even/solution.md

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+
+
+```js run demo
+for (let i = 2; i <= 10; i++) {
+  if (i % 2 == 0) {
+    alert( i );
+  }
+}
+```
+
+We use the "modulo" operator `%` to get the remainder and check for the evenness here.

1-js/02-first-steps/13-while-for/4-for-even/task.md

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+importance: 5
+
+---
+
+# Output even numbers in the loop
+
+Use the `for` loop to output even numbers from `2` to `10`.
+
+[demo]