add subtitle:offset event

packages/artplayer-plugin-subtitle/src/index.js

@@ -61,6 +61,7 @@ function artplayerPluginSubtitle(art) {
                 cue.endTime = cuesCache[index].endTime + time;
             });
             notice.show(`${i18n.get('Subtitle offset time')}: ${value}s`);
+            art.emit('subtitle:offset', value);
         },
     };
 }