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

1-js/02-first-steps/15-function-basics/3-min/solution.md

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+A solution using `if`:
+
+```js
+function min(a, b) {
+  if (a < b) {
+    return a;
+  } else {
+    return b;
+  }
+}
+```
+
+A solution with a question mark operator `'?'`:
+
+```js
+function min(a, b) {
+  return a < b ? a : b;
+}
+```
+
+P.S. In the case of an equality `a == b` it does not matter what to return.

1-js/02-first-steps/15-function-basics/3-min/task.md

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+importance: 1
+
+---
+
+# Function min(a, b)
+
+Write a function `min(a,b)` which returns the least of two numbers `a` and `b`.
+
+For instance:
+
+```js
+min(2, 5) == 2
+min(3, -1) == -1
+min(1, 1) == 1
+```
+