refactor: update export syntax to default for ASR and VAST plugins

docs/assets/ts/artplayer-plugin-asr.d.ts

@@ -1,6 +1,3 @@
-export = artplayerPluginAsr
-export as namespace artplayerPluginAsr;
-
 interface AudioChunk {
   pcm: ArrayBuffer
   wav: ArrayBuffer
@@ -23,5 +20,7 @@ interface AsrPluginInstance {
 
 declare function artplayerPluginAsr(option?: AsrPluginOption): (art: Artplayer) => AsrPluginInstance
 
+export default artplayerPluginAsr
+
 export = artplayerPluginAsr;
 export as namespace artplayerPluginAsr;