[feature request] Designate game to run in a 32 bit prefix with proton

[ghost]

Several games must be run in a 32-bit wine prefix in order to work properly. It would be great if there was an option to set a game to use a 32-bit prefix from within steam.

[kisak-valve]

Prerequisite: ValveSoftware/Proton#449 ValveSoftware/Proton#1087

[hw-claudio]

this would be necessary to get the new re-release of Fallen Earth Classic to work (currently it can't, because FE update requires basically winetricks ie6, which requires a 32bit prefix).