feat(bt): Add entity-based task factories

Add `leaf`, `sequential`, `parallel`, `selector`, `random`, and `repeat`
functions that return `EntityDef` trees. Legacy object definitions are
still
accepted for backward compatibility. Non-task child entities are ignored
by
the runner, and `buildTree` now stores the root entity on
`TaskRunner.root`.

src/index.ts

@@ -8,6 +8,8 @@ export { query } from "./query";
 export { Query } from "./query";
 export type { Entity } from "./entity";
 export { makeEntity, entityIndex, entityGeneration } from "./entity";
+export { entity, buildEntityTree } from "./entity-tree";
+export type { EntityDef, EntityDefChild } from "./entity-tree";
 export { SparseSet } from "./storage/sparse-set";
 export type {
   WorldEvent,