Chapter XVII: Corise of the Coroutines

Replaced update monolithic logic for selection with coroutines because they are amazing and I think they is more how I think than I thought code could be.

Moved Prefabs and Scripts into their own folders for as a sort of naive attempt at organization, just so i don't see them all at once. No worries if someone else changes the organization later, just taking a brute-force first pass.

Added some stupid textures and materials just as a test of some stuff, they can be pulled off of the terrain or deleted outright, for all I care.

Added CTRL-based toggling to selection, and add Shift-to-add support to drag-box selection (https://github.com/rusticgames/rts/issues/1).