Suggestion. Task for 'Iterables' article

還沒有人認領這個 Issue。

It would be useful to have some practice tasks with realization of concepts in article [Iterables](https://javascript.info/iterable).

An idea of task is in the article: "make an iterable object that generates an infinite sequence of pseudorandom numbers".

Possible solution:
```
let randomObj = {
min: 10,
max: 100,
}

randomObj[Symbol.iterator] = function() {
return {
start: this.min,
end: this.max,
next() {
return {
done: false,
value: Math.random() * (this.end - this.start) + this.start,
};
}
}
}

const THRESHOLD = 20;

for (let value of randomObj) {
alert(value); // show random generated value
if (value < THRESHOLD) break;
}
````

先閱讀連結的 Iterables 文章及其現有的練習結構。以提議的無限偽隨機數 iterable 作為任務參考，然後檢查教學課程的其他部分如何組織實作任務。完成的標準是：文章包含適當的實作任務，並提供清楚的題目說明和解決方案指引。

由索引模型根據 Issue 內容生成。