Adding gamemoderun to unix launch script

[noah1510]

This is the first part of adding gamemode support (#950). This PR adds gamemoderun to the unix launch script. I used the modified script on my laptop (AMD Ryzen 7 3700U, Kubuntu 23.10) for at least 30 hours of gameplay and never encountered any issues. There is already a fallback in case gamemode is not installed on the system. It should work just fine, but i only tested it by negating the if operation not on a system without gamemode.

The second part would be adding a default config file, which is not part of this PR.