migrate-directory-tree-v1-to-v2

Use when upgrading a project from directory_tree 1.x to 2.0.0, or fixing analyzer errors after that bump - TreeBuilder/TreeData/FlattenStrategy/VisibleNode/ExpansionSet/SelectionSet/diffVisibleNodes/compileFilter/folderSelection no longer existing, the dart fix renames (removeIndicesDesc, insertIndicesAsc, TreeEntry.fullPath), string node ids becoming NodeId canonical paths, the removed synthetic root and tree_root nodes, case-sensitive paths by default, immutable expansion and selection state, flatten expanding everything by default, natural sibling order, the new search syntax, or tests whose output changed after the upgrade.