Add IsDesktopPlayer function (#feature-request #quest59)

```xml
  <function name="IsDesktopPlayer" type="boolean">
    return (StartsWith (GetFileURL (""), "quest"))
  </function>
```

When using the desktop player, `GetFileURL("")` will return `quest://local/?c=` (plus cache buster numbers)

https://textadventures.co.uk/forum/quest/topic/4995/is-there-a-way-for-the-game-to-check-whether-its-being-played-online-or-not#34125