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

1-js/02-first-steps/13-while-for/1-loop-last-value/solution.md

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+The answer: `1`.
+
+```js run
+let i = 3;
+
+while (i) {
+  alert( i-- );
+}
+```
+
+Every loop iteration decreases `i` by `1`. The check `while(i)` stops the loop when `i = 0`.
+
+Hence, the steps of the loop form the following sequence ("loop unrolled"):
+
+```js
+let i = 3;
+
+alert(i--); // shows 3, decreases i to 2
+
+alert(i--) // shows 2, decreases i to 1
+
+alert(i--) // shows 1, decreases i to 0
+
+// done, while(i) check stops the loop
+```