What problem does this feature solve

devui的tree组件，选中节点的类名目前加在devui-tree__node-content上，希望父级节点devui-tree__node也能添加一个选中的类名，扩展我们对选中父级节点的样式修改

What does the proposed API look like

devui的tree组件，选中节点的类名只加在子节点devui-tree__node-content上