Build a framework for a maintainable combat plugin

* Uses a hierarchy of WeaponClasses -> Weapons for performing attacks,
  with WeaponClass having a set of styles and associated Attacks for
  those styles. Weapons and their classes are built with an easy to use
  and clean DSL.

* Adds a BonusContainer mixin, so that Equipment, Weapons, and
  WeaponClasses can all have their own set of bonuses which apply to the
  player.

* Allows attacks to be queued to the Mobs CombatState instance from
  external code, allowing e.g., NPCs or auto-cast to queue attacks to be
  executed.

data/plugins/combat/projectile.rb

@@ -0,0 +1,3 @@
+def create_projectile(item, drop_rate:, graphic:, projectile:)
+  items = i
+end