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    The `watch(fn, options?)` signature has been replaced by the new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/commits.html

@@ -22,7 +22,7 @@
 </div>
 
 <script>
-const { createApp, ref, watch } = Vue
+const { createApp, ref, watchEffect } = Vue
 const API_URL = `https://api.github.com/repos/vuejs/vue-next/commits?per_page=3&sha=`
 
 const truncate = v => {
@@ -37,7 +37,7 @@ createApp({
     const currentBranch = ref('master')
     const commits = ref(null)
 
-    watch(() => {
+    watchEffect(() => {
       fetch(`${API_URL}${currentBranch.value}`)
         .then(res => res.json())
         .then(data => {