Add main menu

[lunacys]

A simple main menu is nice to have, but more importantly this will allow us to properly learn system run conditions and states.
Useful links:

• https://bevy-cheatbook.github.io/programming/run-criteria.html
• https://bevy-cheatbook.github.io/programming/states.html
• https://github.com/tbillington/bevy_best_practices?tab=readme-ov-file#system-scheduling