feat(runtime-core): add watchEffect API

BREAKING CHANGE: replae `watch(fn, options?)` with `watchEffect`

    The `watch(fn, options?)` signature has been replaced by the new
    `watchEffect` API, which has the same usage and behavior. `watch`
    now only supports the `watch(source, cb, options?)` signautre.

packages/vue/examples/composition/todomvc.html

@@ -53,7 +53,7 @@
 </div>
 
 <script>
-const { createApp, reactive, computed, watch, onMounted, onUnmounted } = Vue
+const { createApp, reactive, computed, watchEffect, onMounted, onUnmounted } = Vue
 
 const STORAGE_KEY = 'todos-vuejs-3.x'
 const todoStorage = {
@@ -119,7 +119,7 @@ createApp({
       })
     })
 
-    watch(() => {
+    watchEffect(() => {
       todoStorage.save(state.todos)
     })