Javascript: Add iterable instance to Diff for destructuring.

Resolves google#39

It can be useful to extract `Diff` elements using destructuring
assignment. For example:

```js
const a = dmp.diff_main('abc', 'ab123c', false);
const [eq, str] = a[0];
```

Because the `Diff` object is not an array, however, this is not
possible, even though it acts like an array.

In this patch a new `Symbol.iterator` method is added to the `Diff`
class so that this pattern can be used.

Props: @JackuB, @GrosSacASac, @TheSpyder
Co-authored-by: Cyril Walle <[email protected]>