chore: use explicit type import to avoid Rollup warning

Author: yyx990803

packages/reactivity/src/reactive.ts

@@ -11,7 +11,7 @@ import {
   shallowCollectionHandlers,
   shallowReadonlyCollectionHandlers
 } from './collectionHandlers'
-import { UnwrapRefSimple, Ref, RawSymbol } from './ref'
+import type { UnwrapRefSimple, Ref, RawSymbol } from './ref'
 
 export const enum ReactiveFlags {
   SKIP = '__v_skip',