Fix bfcache with persistent atom and map and added "bf cache behaviour" playwright tests

1) Fixed this issue https://github.com/nanostores/persistent/issues/35 I simply added "sync" function and "pageshow" listener in "windowPersistentEvents" (I'm not sure if typing of "PersistentListener" should be updated because this would only be available in "windowPersistentEvents") 2) Added playwright tests to bfcache. 3) Added "demo" folder with simple vite app to test bf cache by hand if needed. 4) Increased total size a bit (less than 50 bytes). This can probably be improved if needed.