Implement the main menu (closes #28)

Assets/Prefabs.meta

Assets/Scenes/MainMenu.unity

Assets/Scenes/MainMenu.unity.meta

Assets/Scenes/Simulation.unity

Assets/Scenes/Simulation.unity.meta

Assets/Scripts/Presentation.meta

Assets/Scripts/Presentation/Presentation.asmdef

Assets/Scripts/Presentation/Presentation.asmdef.meta

Assets/Scripts/Presentation/UI.meta

Assets/Scripts/Presentation/UI/MainMenu.cs

Assets/Scripts/Presentation/UI/MainMenu.cs.meta

Assets/Scripts/Presentation/World.meta

Assets/Scripts/Presentation/World/GameManager.cs

Assets/Scripts/Presentation/World/GameManager.cs.meta

Assets/UI Toolkit.meta

Assets/UI Toolkit/PanelSettings.asset

Assets/UI Toolkit/PanelSettings.asset.meta

Assets/UI Toolkit/UnityThemes.meta

Assets/UI Toolkit/UnityThemes/UnityDefaultRuntimeTheme.tss

.../UI Toolkit/UnityThemes/UnityDefaultRuntimeTheme.tss.meta