chore: 删除不再使用的 TypeScript 声明文件，并添加新的插件声明文件

docs/assets/ts/artplayer-plugin-auto-thumbnail.d.ts

@@ -0,0 +1,17 @@
+import type Artplayer from 'artplayer';
+
+export = artplayerPluginAutoThumbnail;
+export as namespace artplayerPluginAutoThumbnail;
+
+type Option = {
+    url?: string;
+    width?: number;
+    height?: number;
+    scale?: number;
+};
+
+type Result = {
+    name: 'artplayerPluginAutoThumbnail';
+};
+
+declare const artplayerPluginAutoThumbnail: (option: Option) => (art: Artplayer) => Result;