Refactor(LegacyPromotions): Explicitly load state machine decorator

Solidus Support's new Zeitwerk-based loading mechanism only works for
decorating autoloaded classes. The order state machine is in `lib`, and
thus is not autoloaded, so this decorator is not loaded either unless we explicitly
load it.

legacy_promotions/lib/solidus_legacy_promotions/engine.rb

@@ -6,6 +6,10 @@ module SolidusLegacyPromotions
   class Engine < ::Rails::Engine
     include SolidusSupport::EngineExtensions
 
+    initializer "solidus_legacy_promotions.patch_state_machine", after: "spree.load_config_initializers" do
+      config.to_prepare { SolidusLegacyPromotions::SpreeOrderStateMachineDecorator }
+    end
+
     initializer "solidus_legacy_promotions.add_backend_menu_item" do
       if SolidusSupport.backend_available?
         promotions_menu_item = Spree::BackendConfiguration::MenuItem.new(