fix(customElement): keep appContext during custom element update

Author: edison1105

packages/runtime-dom/src/apiCustomElement.ts

@@ -520,7 +520,9 @@ export class VueElement
   }
 
   private _update() {
-    render(this._createVNode(), this._root)
+    const vnode = this._createVNode()
+    if (this._app) vnode.appContext = this._app._context
+    render(vnode, this._root)
   }
 
   private _createVNode(): VNode<any, any> {
@@ -573,6 +575,7 @@ export class VueElement
         this._setParent()
       }
     }
+
     return vnode
   }