feat($theme-default) Propagate toggleSidebar event to parent (#1816)

Author: shortdiv

packages/@vuepress/theme-default/layouts/Layout.vue

@@ -121,6 +121,7 @@ export default {
   methods: {
     toggleSidebar (to) {
       this.isSidebarOpen = typeof to === 'boolean' ? to : !this.isSidebarOpen
+      this.$emit('toggle-sidebar', this.isSidebarOpen)
     },
 
     // side swipe