fix(NMenu): The level of the sub-menu is not automatically expanded

[cnguu]

TuSimple/naive-ui version (版本)

2.15.4

Vue version (Vue 版本)

3.1.4

Browser and its version (浏览器及其版本)

Chrome(91.0.4472.124)

System and its version (系统及其版本)

Window10 20H2(19042.1083)

Node version (Node 版本)

Reappearance link (重现链接)

https://codesandbox.io/s/intelligent-ellis-k3oew?file=/src/Content.vue

Reappearance steps (重现步骤)

Open it and you will find

Expected results (期望的结果)

Select the submenu to automatically expand its level

Actual results (实际的结果)

The selected submenu does not automatically expand its level

Remarks (补充说明)