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/assets/example/pip.js

@@ -0,0 +1,18 @@
+// npm i artplayer-plugin-pip
+// import artplayerPluginPip from 'artplayer-plugin-pip';
+
+let art = new Artplayer({
+  container: '.artplayer-app',
+  url: '/assets/sample/video.mp4',
+  plugins: [
+    artplayerPluginPip({
+      width: 480,
+      height: 270,
+      placeholder: `Playing in Document Picture-in-Picture`,
+    }),
+  ],
+})
+
+art.on('document-pip', (state) => {
+  console.log('Document Picture-in-Picture', state)
+})