Add Proton-GE stable Flatpak as runner

[deadmeu]

When setting up the Proton GE runner, it seems like currently you can only select to download and install a specific version of ge-proton. It would be nice if, additionally, you could specify to use the prepackaged Flatpak (com.valvesoftware.Steam.CompatibilityTool.Proton-GE) as a runner so that it is automatically kept up-to-date.