Replace the deprecated unload event with pagehide

The unloadHandler is called by an eventListener for the 'unload' event when the user
moves away from the page being served by y-websocket.  However, the unload event is now
deprecated (see https://web.dev/articles/bfcache?utm_source=lighthouse&utm_medium=devtools#never-use-the-unload-event )
and the recommendation is to replace it with the 'pagehide' event.  This PR does that, in both locations
in y-websocket.js where it is used.

src/y-websocket.js

@@ -358,7 +358,7 @@ export class WebsocketProvider extends Observable {
       )
     }
     if (typeof window !== 'undefined') {
-      window.addEventListener('unload', this._unloadHandler)
+      window.addEventListener('pagehide', this._unloadHandler)
     } else if (typeof process !== 'undefined') {
       process.on('exit', this._unloadHandler)
     }
@@ -401,7 +401,7 @@ export class WebsocketProvider extends Observable {
     clearInterval(this._checkInterval)
     this.disconnect()
     if (typeof window !== 'undefined') {
-      window.removeEventListener('unload', this._unloadHandler)
+      window.removeEventListener('pagehide', this._unloadHandler)
     } else if (typeof process !== 'undefined') {
       process.off('exit', this._unloadHandler)
     }