[Linux] [Improvement] Setting to point to Steam VR executable

New to contributing, sorry for any mistakes!

Pretty simple, it would be nice to set the steam vr command in the settings. It would be good for laptops with hybrid graphics (like mine) to be able to set a command prefix to run on the dedicated gpu rather than the integrated. Also if you have multiple different steam installations (native vs flathub) you could specify which one to use.

That's it! Do with this what you will!