[Carousel] Wrapper around items

## Bug Report

**Current Behavior**
Right now there is no wrapper around `cmp-carousel__item` and because of that is impossible to use a third-party library to create a carousel.

**Expected behavior/code**
`div` around `cmp-carousel__item`.

**Environment**
- Core Components version 2.3.3-SNAPSHOT

**Possible Solution**
```

Start by locating the carousel markup that renders cmp-carousel__item and compare it with the proposed data-sly-repeat structure. Add the cmp-carousel__items wrapper around the repeated items, then verify that every item remains rendered and the resulting structure supports third-party carousel libraries.

Written by the indexing model from the issue text.