Feature Request: allow items reordering

[SeriousMonk]

Hi, thank you for this package it's truly amazing!

I would like to suggest a new feature. It would be relly cool if this package could suppport item reorder using drag and drop. Basically the user should be able to pick up a node of the tree and drag and drop it anywhere else in the tree at the same level.

What I'm suggesting is a behavior similar to this, but adapted to work on a tree structure:

There currently is no package that supports item reordering on more than 2 levels, and I believe this package has the structure to support this freature without any crazy work necessary, since it is already based on ReorderableListView.

I created a fork and started working on this, but my skills and understanding of the structure of this package are insuficcient to complete the task.