Mods and mod loader

[olefasting]

Mod loading has been implemented (#366) so what remains is to create a mod manager in launcher.

For now we only support data mods but we will add script support in the future.

We should also implement mods that replace all core resources (currently, mods extend this), for when people want to create total conversions.

[zicklag]

The bevy rewrite started to move towards "everything is an asset" which allowed the complete conversion of the game, and easy modification if you wanted to just manually modify the asset folder, but not a true mod loading workflow.

We do have a scripting system now, that is temporarily disabled because they couldn't perform fast enough to work for multiplayer games.

We should at least be able to use scripts for local mods, but we'll need to do more exploration as far as improving performance or alternative modding strategies as we move forward.