fix(tree): fix the MatTreeNodeOutlet not exported issue (#10581)

Author: tinayuangao

src/lib/tree/public-api.ts

@@ -12,5 +12,6 @@ export * from './padding';
 export * from './tree';
 export * from './tree-module';
 export * from './toggle';
+export * from './outlet';
 export * from './data-source/flat-data-source';
 export * from './data-source/nested-data-source';