fix(types): missing types for nested sidebar group (close: #3127)

Currently we config config nested sub group<sup>[1]</sup>, but the types are missing:

[1] https://vuepress.vuejs.org/theme/default-theme-config.html#sidebar-groups

Author: ulivz

packages/@vuepress/types/src/theme-default.ts

@@ -84,7 +84,7 @@ export type SidebarItem4Group = {
   /**
    * Sidebar children.
    */
-  children?: Array<SidebarItem4Shortcut>;
+  children?: Array<SidebarItem4Shortcut | SidebarItem4Group>;
 };
 
 /**