add update method to placeholder

packages/tiptap-extensions/src/extensions/Placeholder.js

@@ -15,6 +15,13 @@ export default class Placeholder extends Extension {
     }
   }
 
+  get update() {
+    return ({ state, view }) => {
+      // TODO: fix error when content is not empty
+      view.updateState(state)
+    }
+  }
+
   get plugins() {
     return [
       new Plugin({