Drag & drop from chat UI available as standalone component

### Issue type

**I'm submitting a ...**

* [ ] bug report
* [X] feature request

### Issue description

**Current behavior:**
Drag & drop is implemented in Chat UI, but is not available as a standalone component.

**Expected behavior:**
It would be very nice to have a standalone module/component (e.g. `NbDragAndDrop`) to easily upload files.

The component could emit an event on file(s) selection.

It would be very handy to allow `multiple` files uploads.