Transform [].forEach => for-in

## Is your feature request related to a problem? Please describe.
Many `[].forEach` calls can be converted to a `for...in` safely.

```javascript
const LANGUAGES = { javascript, json, xml };
Object.keys(LANGUAGES).forEach( key => hljs.registerLanguage(key, LANGUAGES[key]) );
```

Can be converted to...
```javascript
const LANGUAGES = { javascript, json, xml };
for (key in LANGUAGES) { hljs.registerLanguage(key, LANGUAGES[key]) }
```

## Describe the solution you'd like
This transform can be done safely so long as the inner function doesn't rely on `this` being auto bound.

It's worth investigating if there are cases where a function call can be generated that passes the scope as an argument and leverages it correctly as well.

## Describe alternatives you've considered
Ignore this transform entirely.

**Edit**: If order is important, `for...of` is a better choice.
>Because the order of iteration is implementation-dependent, iterating over an array may not visit elements in a consistent order. Therefore, it is better to use a for loop with a numeric index (or Array.prototype.forEach() or the for...of loop) when iterating over arrays where the order of access is important.