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

1-js/02-first-steps/08-operators/4-fix-prompt/task.md

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+importance: 5
+
+---
+
+# Fix the addition
+
+Here's a code that asks the user for two numbers and shows their sum.
+
+It works incorrectly. The output in the example below is `12` (for default prompt values).
+
+Why? Fix it. The result should be `3`.
+
+```js run
+let a = prompt("First number?", 1);
+let b = prompt("Second number?", 2);
+
+alert(a + b); // 12
+```