refactor: remove use of Object.assign

TS already transpiles spread to Object.assign with target:es2016

packages/reactivity/src/effect.ts

@@ -237,14 +237,10 @@ export function trackEffects(
     dep.add(activeEffect!)
     activeEffect!.deps.push(dep)
     if (__DEV__ && activeEffect!.onTrack) {
-      activeEffect!.onTrack(
-        Object.assign(
-          {
-            effect: activeEffect!
-          },
-          debuggerEventExtraInfo
-        )
-      )
+      activeEffect!.onTrack({
+        effect: activeEffect!,
+        ...debuggerEventExtraInfo!
+      })
     }
   }
 }

packages/runtime-core/src/apiCreateApp.ts

@@ -179,7 +179,6 @@ export function createAppAPI<HostElement>(
   hydrate?: RootHydrateFunction
 ): CreateAppFunction<HostElement> {
   return function createApp(rootComponent, rootProps = null) {
-
     if (!isFunction(rootComponent)) {
       rootComponent = { ...rootComponent }
     }

packages/runtime-core/src/apiWatch.ts

@@ -92,7 +92,7 @@ export function watchPostEffect(
     effect,
     null,
     (__DEV__
-      ? Object.assign(options || {}, { flush: 'post' })
+      ? { ...options, flush: 'post' }
       : { flush: 'post' }) as WatchOptionsBase
   )
 }
@@ -105,7 +105,7 @@ export function watchSyncEffect(
     effect,
     null,
     (__DEV__
-      ? Object.assign(options || {}, { flush: 'sync' })
+      ? { ...options, flush: 'sync' }
       : { flush: 'sync' }) as WatchOptionsBase
   )
 }