chore: restructure vue-sync-engine workspace and remove unused files

vue-sync-engine/lib/src/composables/useEntity.ts

@@ -0,0 +1,15 @@
+import { computed, type ComputedRef, type MaybeRefOrGetter, toValue } from 'vue'
+import type { EntityDef, EntityId } from '../core/types'
+import { useEngine } from './useEngine'
+
+export function useEntity<T>(
+  def: EntityDef<T>,
+  id: MaybeRefOrGetter<EntityId | undefined>,
+): ComputedRef<T | undefined> {
+  const engine = useEngine()
+  return computed(() => {
+    const v = toValue(id)
+    if (v === undefined || v === null) return undefined
+    return engine.mirror.getEntity<T>(def.name, v)
+  })
+}