feat: add Document Picture-in-Picture plugin for ArtPlayer

- Implemented the core functionality of the Document Picture-in-Picture plugin.
- Created main plugin file `artplayer-plugin-document-pip.js` with support for opening and closing the PIP window.
- Added legacy support in `artplayer-plugin-document-pip.legacy.js`.
- Defined plugin options in `package.json` and included necessary metadata.
- Developed the main logic in `src/index.js`, including style copying and event handling.
- Created TypeScript definitions in `types/artplayer-plugin-document-pip.d.ts` for better type safety.

docs/assets/example/document.pip.js

@@ -0,0 +1,19 @@
+// npm i artplayer-plugin-document-pip
+// import artplayerPluginDocumentPip from 'artplayer-plugin-document-pip';
+
+let art = new Artplayer({
+  container: '.artplayer-app',
+  url: '/assets/sample/video.mp4',
+  plugins: [
+    artplayerPluginDocumentPip({
+      width: 480,
+      height: 270,
+      fallbackToVideoPiP: true,
+      placeholder: `Playing in Document Picture-in-Picture`,
+    }),
+  ],
+})
+
+art.on('document-pip', (state) => {
+  console.log('Document Picture-in-Picture', state)
+})