Add StartlevelEvent (#294)

lib/openhab/core/events/startlevel_event.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+# @deprecated OH3.4 this guard is not needed on OH4
+return unless OpenHAB::Core.version >= OpenHAB::Core::V4_0
+
+module OpenHAB
+  module Core
+    module Events
+      java_import org.openhab.core.events.system.StartlevelEvent
+
+      #
+      # The {AbstractEvent} sent when the system start level changed.
+      #
+      # @!attribute [r] startlevel
+      #   @return [Integer] The new start level.
+      #
+      class StartlevelEvent < AbstractEvent; end
+    end
+  end
+end