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

1-js/02-first-steps/08-operators/2-assignment-result/solution.md

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+The answer is:
+
+- `a = 4` (multiplied by 2)
+- `x = 5` (calculated as 1 + 4)
+

1-js/02-first-steps/08-operators/2-assignment-result/task.md

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+importance: 3
+
+---
+
+# Assignment result
+
+What are the values of `a` and `x` after the code below?
+
+```js
+let a = 2;
+
+let x = 1 + (a *= 2);
+```