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.

docs/index.html

@@ -111,6 +111,9 @@
                             <a
                                 href="/?libs=./uncompiled/artplayer-plugin-asr/index.js&example=asr">artplayer-plugin-asr</a>
                         </li>
+                        <li>
+                            <a href="/?libs=./uncompiled/artplayer-plugin-pip/index.js&example=pip">artplayer-plugin-pip</a>
+                        </li>
                     </ul>
                     <ul>
                         <li class="title">Proxys</li>