add demo from figma plugin example

src/App.vue

@@ -1,12 +1,12 @@
 <script setup lang="ts">
-// This starter template is using Vue 3 <script setup> SFCs
-// Check out https://v3.vuejs.org/api/sfc-script-setup.html#sfc-script-setup
 import HelloWorld from './components/HelloWorld.vue'
+import RectangleCreatorDemo from './components/RectangleCreatorDemo.vue'
 </script>
 
 <template>
   <img alt="Vue logo" src="./assets/logo.png" />
   <HelloWorld msg="Hello Vue 3 + TypeScript + Vite" />
+  <RectangleCreatorDemo />
 </template>
 
 <style>

src/components/RectangleCreatorDemo.vue

@@ -0,0 +1,19 @@
+<script setup lang="ts">
+import { ref } from 'vue'
+const count = ref(0)
+
+const create = () => {
+  parent.postMessage({ pluginMessage: { type: 'create-rectangles', count: count.value } }, '*')
+}
+
+const cancel = () => {
+  parent.postMessage({ pluginMessage: { type: 'cancel' } }, '*')
+}
+</script>
+
+<template>
+  <h2>Rectangle Creator</h2>
+  <p>Count: <input id="count" v-model="count"></p>
+  <button @click="create">Create</button>
+  <button @click="cancel">Cancel</button>
+</template>