fix(runtime-dom): v-cloak should be removed after compile on the root element (#893)

fix #890

packages/runtime-dom/__tests__/directives/vCloak.spec.ts

@@ -0,0 +1,10 @@
+import { createApp } from '@vue/runtime-dom'
+
+describe('vCloak', () => {
+  test('should be removed after compile', () => {
+    const root = document.createElement('div')
+    root.setAttribute('v-cloak', '')
+    createApp({}).mount(root)
+    expect(root.hasAttribute('v-cloak')).toBe(false)
+  })
+})