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    The `watch(fn, options?)` signature has been replaced by the new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/runtime-core/__tests__/components/Suspense.spec.ts

@@ -9,6 +9,7 @@ import {
   nextTick,
   onMounted,
   watch,
+  watchEffect,
   onUnmounted,
   onErrorCaptured
 } from '@vue/runtime-test'
@@ -163,7 +164,7 @@ describe('Suspense', () => {
         // extra tick needed for Node 12+
         deps.push(p.then(() => Promise.resolve()))
 
-        watch(() => {
+        watchEffect(() => {
           calls.push('immediate effect')
         })
 
@@ -265,7 +266,7 @@ describe('Suspense', () => {
         const p = new Promise(r => setTimeout(r, 1))
         deps.push(p)
 
-        watch(() => {
+        watchEffect(() => {
           calls.push('immediate effect')
         })