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

1-js/02-first-steps/13-while-for/3-which-value-for/solution.md

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+**The answer: from `0` to `4` in both cases.**
+
+```js run
+for (let i = 0; i < 5; ++i) alert( i );
+
+for (let i = 0; i < 5; i++) alert( i );
+```
+
+That can be easily deducted from the algorithm of `for`:
+
+1. Execute once `i = 0` before everything (begin).
+2. Check the condition `i < 5`
+3. If `true` -- execute the loop body `alert(i)`, and then `i++`
+
+The increment `i++` is separated from the condition check (2). That's just another statement.
+
+The value returned by the increment is not used here, so there's no difference between `i++` and `++i`.

1-js/02-first-steps/13-while-for/3-which-value-for/task.md

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+importance: 4
+
+---
+
+# Which values get shown by the "for" loop?
+
+For each loop write down which values it is going to show. Then compare with the answer.
+
+Both loops `alert` same values or not?
+
+1. The postfix form:
+
+    ```js
+    for (let i = 0; i < 5; i++) alert( i );
+    ```
+2. The prefix form:
+
+    ```js
+    for (let i = 0; i < 5; ++i) alert( i );
+    ```