feat(reactivity): add support for customRef API

packages/reactivity/__tests__/ref.spec.ts

@@ -8,7 +8,7 @@ import {
   isReactive
 } from '../src/index'
 import { computed } from '@vue/runtime-dom'
-import { shallowRef, unref } from '../src/ref'
+import { shallowRef, unref, customRef } from '../src/ref'
 
 describe('reactivity/ref', () => {
   it('should hold a value', () => {
@@ -208,4 +208,33 @@ describe('reactivity/ref', () => {
     expect(dummyX).toBe(4)
     expect(dummyY).toBe(5)
   })
+
+  test('customRef', () => {
+    let value = 1
+    let _trigger: () => void
+
+    const custom = customRef((track, trigger) => ({
+      get() {
+        track()
+        return value
+      },
+      set(newValue: number) {
+        value = newValue
+        _trigger = trigger
+      }
+    }))
+
+    let dummy
+    effect(() => {
+      dummy = custom.value
+    })
+    expect(dummy).toBe(1)
+
+    custom.value = 2
+    // should not trigger yet
+    expect(dummy).toBe(1)
+
+    _trigger!()
+    expect(dummy).toBe(2)
+  })
 })