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

1-js/02-first-steps/13-while-for/2-which-value-while/task.md

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+importance: 4
+
+---
+
+# Which values does the while loop show?
+
+For every loop iteration, write down which value it outputs and then compare it with the solution.
+
+Both loops `alert` the same values, or not?
+
+1. The prefix form `++i`:
+
+    ```js
+    let i = 0;
+    while (++i < 5) alert( i );
+    ```
+2. The postfix form `i++`
+
+    ```js
+    let i = 0;
+    while (i++ < 5) alert( i );
+    ```