feat(watch): 实现watch功能并完善响应式系统

- 新增apiWatch.ts实现watch功能，支持immediate和deep选项
- 扩展ReactiveEffect类添加stop方法用于停止监听
- 导出ReactiveEffect和isReactive函数供外部使用
- 添加ReactiveFlags枚举和IS_REACTIVE标识符
- 在reactive对象上添加__v_isReactive标识
- 导出EMPTY_OBJ常量用于默认参数
- 添加watch功能到Vue入口文件
- 创建watch.html示例验证监听功能正常工作

packages/reactivity/src/effect.ts

@@ -41,6 +41,7 @@ export class ReactiveEffect<T = any> {
     activeEffect = this
     return this.fn() //完成第一次getter行为的触发
   }
+  stop() {}
 }
 
 /**

packages/reactivity/src/index.ts

@@ -1,4 +1,4 @@
-export { reactive } from './reactive'
-export { effect } from './effect'
+export { reactive, isReactive } from './reactive'
+export { effect, ReactiveEffect } from './effect'
 export { ref } from './ref'
 export { computed } from './computed'

packages/reactivity/src/reactive.ts

@@ -8,6 +8,10 @@ import { isObject } from '@vue/shared'
  */
 export const reactiveMap = new WeakMap<object, any>()
 
+export const enum ReactiveFlags {
+  IS_REACTIVE = '__v_isReactive'
+}
+
 /**
  * 为复杂数据类型, 创建响应式对象
  * @param target 被代理的对象
@@ -35,7 +39,8 @@ function createReactiveObject(
   }
   //未被代理则生成proxy实例
   const proxy = new Proxy(target, baseHandlers)
-
+  //添加reactive标识
+  proxy[ReactiveFlags.IS_REACTIVE] = true
   //缓存代理对象
   proxyMap.set(target, proxy)
   return proxy
@@ -43,3 +48,7 @@ function createReactiveObject(
 
 export const toReactive = <T extends unknown>(value: T): T =>
   isObject(value) ? reactive(value as object) : value
+
+export function isReactive(value): boolean {
+  return !!(value && value[ReactiveFlags.IS_REACTIVE])
+}