fix #13185 - keyboard support for carousel

This seems more fitting :smile:

```js
Carousel.prototype.keydown = function (e) {
  if (!/37|39/.test(e.which)) return
  
  this[e.which == 37 ? 'prev' : 'next']()

  e.preventDefault()
}
```

Also, [ref](https://github.com/twbs/bootstrap/blob/c0e82accadea85094ef2f8b208975c449130f33f/js/dropdown.js#L60)