Support for Pressure-Vessel (Steam Linux Runtime) Applications

Through doing some playtesting and tinkering with a variety of Linux-native applications, I have noticed that if using a SLR compatibility layer for native titles, Steam will prepend a very similar launch stanza to the game launch, making it difficult to prepend multiple tools. Getting a custom-launch/multi-launch for native titles through Steam likely isn't as major as getting everything to work inside the same Proton prefix, but could still be useful if a particular game requires stale libraries that aren't packaged on modern distros. In particular, being able to custom-launch embedded/launcher modding utilities and similar applications for certain games could warrant a use-case for adding this feature.