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    The `watch(fn, options?)` signature has been replaced by the new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

@@ -71,6 +71,14 @@ export type StopHandle = () => void
 
 const invoke = (fn: Function) => fn()
 
+// Simple effect.
+export function watchEffect(
+  effect: WatchEffect,
+  options?: BaseWatchOptions
+): StopHandle {
+  return doWatch(effect, null, options)
+}
+
 // initial value for watchers to trigger on undefined initial values
 const INITIAL_WATCHER_VALUE = {}
 
@@ -110,6 +118,13 @@ export function watch<T = any>(
     // watch(source, cb)
     return doWatch(effectOrSource, cbOrOptions, options)
   } else {
+    // TODO remove this in the next release
+    __DEV__ &&
+      warn(
+        `\`watch(fn, options?)\` signature has been moved to a separate API. ` +
+          `Use \`watchEffect(fn, options?)\` instead. \`watch\` will only ` +
+          `support \`watch(source, cb, options?) signature in the next release.`
+      )
     // watch(effect)
     return doWatch(effectOrSource, null, cbOrOptions)
   }

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,