feat(reactivity): add isShallow api

packages/reactivity/__tests__/ref.spec.ts

@@ -10,6 +10,7 @@ import {
 } from '../src/index'
 import { computed } from '@vue/runtime-dom'
 import { shallowRef, unref, customRef, triggerRef } from '../src/ref'
+import { isShallow } from '../src/reactive'
 
 describe('reactivity/ref', () => {
   it('should hold a value', () => {
@@ -227,6 +228,10 @@ describe('reactivity/ref', () => {
     expect(dummy).toBe(2)
   })
 
+  test('shallowRef isShallow', () => {
+    expect(isShallow(shallowRef({ a: 1 }))).toBe(true)
+  })
+
   test('isRef', () => {
     expect(isRef(ref(1))).toBe(true)
     expect(isRef(computed(() => 1))).toBe(true)