Add onDragStart callback

[lcacheux]

I needed a way to handle long press events for items in a reorderable list with detectReorderAfterLongPress, but using the combinedClickable modifier didn't worked in combination with the event management of ComposeReorderable.

The solution I found was to detect the onDragStart event, but it was not available in the API (unlike the onDragEnd event).

[bpappin]

This PR was copied to another fork. https://github.com/bpappin/ComposeReorderable/pull/4