feat: Refactor thumbnails implementation in Artplayer

This commit refactors the `thumbnails` implementation in Artplayer to improve code readability and maintainability. The code changes remove unnecessary variables and simplify the logic for displaying thumbnails. This refactor enhances the overall performance and user experience of the Artplayer plugin.

packages/artplayer/src/control/thumbnails.js

@@ -2,7 +2,7 @@ export default function thumbnails(options) {
     return (art) => ({
         ...options,
         mounted: () => {
-            this.art.thumbnails = art.options.thumbnails;
+            art.thumbnails = art.options.thumbnails;
         },
     });
 }