feat: 增加字幕相关属性和事件，优化字幕切换逻辑

packages/artplayer/types/artplayer.d.ts

@@ -152,6 +152,9 @@ declare class Artplayer extends Player {
     readonly subtitle: {
         get url(): string;
         set url(url: string);
+        get textTrack(): TextTrack;
+        get activeCue(): VTTCue;
+        get cues(): TextTrackCueList;
         style(name: string | Partial<CSSStyleDeclaration>, value?: string): void;
         switch(url: string, option?: Subtitle): Promise<string>;
     } & Component;

packages/artplayer/types/events.d.ts

@@ -36,9 +36,11 @@ export type Events = {
     setting: [state: boolean];
     hotkey: [event: Event];
     destroy: [];
-    subtitleUpdate: [text: string];
+    subtitleBeforeUpdate: [cue: VTTCue];
+    subtitleAfterUpdate: [cue: VTTCue];
     subtitleLoad: [url: string];
     subtitleSwitch: [url: string];
+    subtitleTrackLoad: [track: HTMLTrackElement, event: Event];
     focus: [event: Event];
     blur: [event: Event];
     dblclick: [event: Event];