Add initial spread of plugins for the essentials of the game

Created a bunch of blank plugins, a single plugin group to register all of our own plugins, and added that group and DefaultPlugins to the App. Additionally, fixed the Nix shell.

As documented in src/plugins/mod.rs

• meta_devux
  • Interfaces/features that exist only for developers working on features.
• meta_persist
  • Any data that needs be persisted, such as configurations and world saves.
• npc_gen
  • When there is a need for one or more NPCs to be generated, this will handle that, ensuring that batches are fairly balanced with their contexts.
• npc_mechs
  • Essentially, the NPCs' AIs and everything that goes with them.
• player_mechs
  • Mechanics that revolve around the player experience. Things that guide, affect, or respond to the player's experience.
• player_ux
  • Camera, HUD, menus, etc. Everything about giving information to the player or taking input from them, but no further.
• terra_gen
  • Generating the world, including NPCs, pre-establshed settlements and events.
• terra_mechs
  • Time, weather, physics, etc.
• terra_ux
  • Worldly interaction hints as well as rendering the AV of the world itself.

Not included for now:

• Server/client dichotomy, or multiplayer