Add a plugin which allows creating mixins for Mobs

Adds a mob-extension plugin, which allows creating modules as mixins to
be monkey-patched into the Mob class.  Having a standard method of doing
this prevents confusing problems with mixin name conflicts, by allowing
the plugin to raise an error that 2 mixins with the same name exist.

data/plugins/entity/mob/extension/extension.rb

@@ -0,0 +1,20 @@
+require 'java'
+
+java_import 'org.apollo.game.model.entity.Mob'
+
+module MobExtension
+  MOB_EXTENSIONS = []
+
+  def self.register(extension)
+    fail 'Provided extension object is not a module' unless extension.is_a?(Module)
+
+    new_mixins = extension.public_instance_methods
+    current_mixins = MOB_EXTENSIONS.map { |e| {e.to_s => e.public_instance_methods} }
+
+    current_mixins.each do |ext, methods|
+      methods.each {|m| fail "Extension #{ext} already provides method #{m}" if new_mixins.include?(m) }
+    end
+
+    Mob.include(extension)
+  end
+end