Support Ref to enable usage with Ref from `react-virtualized`

## Issues
Currently, `UnstableFastTree` does not support Ref. Please support Ref to enable usage with Ref from `react-virtualized`.

![Image](https://github.com/user-attachments/assets/68afa902-3a84-4c64-8f5d-3f1de75a756f)

## Version

"react-virtualized-tree": "^3.4.1"