refactor: improve document Picture-in-Picture support checks and update type definitions

- Renamed variable `dpipOK` to `isSupported` for clarity in checking Document Picture-in-Picture support.
- Updated the `open` function to use the new `isSupported` variable.
- Removed redundant checks in the `close` function related to Document Picture-in-Picture support.
- Added `isSupported` and `isActive` properties to the plugin's result interface in TypeScript definitions.
- Cleaned up the code for better readability and maintainability.

docs/assets/ts/artplayer-plugin-document-pip.d.ts

@@ -7,16 +7,11 @@ interface Option {
 
 interface Result {
   name: 'artplayerPluginDocumentPip'
+  isSupported: boolean
+  isActive: boolean
   open: () => void
   close: () => void
   toggle: () => void
-  state: {
-    win: Window | null
-    originalParent: Element | null
-    originalNext: Element | null
-    currentDoc: Document | null
-    placeholder: Element | null
-  }
 }
 
 declare const artplayerPluginDocumentPip: (option: Option) => (art: Artplayer) => Result