简化工具函数

packages/artplayer/src/player/playingMix.js

@@ -1,6 +1,8 @@
+import { def } from '../utils';
+
 export default function playingMix(art, player) {
     const { $video } = art.template;
-    Object.defineProperty(player, 'playing', {
+    def(player, 'playing', {
         get: () => !!($video.currentTime > 0 && !$video.paused && !$video.ended && $video.readyState > 2),
     });
 }