Option to snap automatically to nearest possible level

[d9k]

Option to snap automatically to nearest possible level if canDrop property defined.

[d9k]

Added options autoSnapRowsAfter, autoSnapRowsBefore

See draft at https://github.com/d9k/react-sortable-tree/tree/feature-11518
Storybook: http://0.0.0.0:3001/?path=/story/advanced--auto-snap-to-level