feat: add danmuku plugin and i18n setup documentation

- Created a new documentation file for the danmuku plugin with code examples.
- Added a new documentation file for language settings (i18n) with detailed instructions on language imports, default languages, and how to add or modify languages.

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

@@ -28,5 +28,5 @@ declare class ArtplayerPluginIframe {
 
 export default ArtplayerPluginIframe
 
-export = artplayerPluginIframe
-export as namespace artplayerPluginIframe;
+export = artplayerPluginIframe;
+export as namespace artplayerPluginIframe;