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    The `watch(fn, options?)` signature has been replaced by the new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/src/index.ts

@@ -17,7 +17,7 @@ export {
   markNonReactive
 } from '@vue/reactivity'
 export { computed } from './apiComputed'
-export { watch } from './apiWatch'
+export { watch, watchEffect } from './apiWatch'
 export {
   onBeforeMount,
   onMounted,