feat: add Picture-in-Picture plugin for ArtPlayer

- Introduced `artplayer-plugin-pip` to enable Picture-in-Picture functionality.
- Updated `artplayer.d.ts` to export new types related to the plugin.
- Added example usage of the plugin in `pip.js`.
- Created TypeScript definitions for the plugin in `artplayer-plugin-pip.d.ts`.
- Implemented the plugin logic in `index.js` with necessary styles and event handling.
- Updated documentation to include the new plugin and its usage.
- Added README and package.json for the new plugin.

packages/artplayer-plugin-pip/README.md

@@ -0,0 +1,11 @@
+# artplayer-plugin-pip
+
+pip plugin for ArtPlayer
+
+## Demo
+
+[https://artplayer.org](https://artplayer.org/?libs=./uncompiled/artplayer-plugin-pip/index.js&example=pip)
+
+## License
+
+MIT © Harvey Zack