migrate-flutter-directory-tree-v1-to-v2

Use when upgrading a Flutter project from flutter_directory_tree 1.x to 2.0, or fixing analyzer errors after that bump - TreeBuilder and TreeData replaced by DirectoryTree, string ids replaced by NodeId paths, the removed ExpansionController, SelectionController, SelectionShortcuts, TreeNodeTile, ContextMenuDelegate, and IconProvider, nodeBuilder to rowBuilder, rebuild and revealAndScroll replacements, the picker's allowMultiple and foldersOnly, the dart fix theme renames (rowHeight, indent), SelectionMode.multi, and behavior changes that compile but do nothing, such as dropped immutable state values.