add `ISteamUtils::IsOvrelayEnabled`

[hajimehoshi]

https://partner.steamgames.com/doc/api/ISteamUtils#IsOverlayEnabled

This is needed to suspend the game. A developer might receive the following feedback:

The following feedback is not strictly required for Steam's gaming support, but reflects some best-practices that we have discovered in the course of testing. While no follow up steps are required, addressing these issues will improve the game play experience. - When opening the Steam Overlay, by pressing the home button on the controller, the game does not pause and continues to function while navigating the Steam Overlay menu. - When the controller is unplugged during gameplay, the game does not automatically pause. - For both the "Full" and "Partial" Controller Support category that appears on the store page, it's expected to have a "Developer's Recommended Configuration." For more information on how to create and publish a controller configuration, please see: https://partner.steamgames.com/doc/features/steam_controller/getting_started_for_devs We recommend resolving these issues, but they will not prevent your game from releasing.