Add basic sprockets support

Add hotwire_spark.js to the precompiled assets. This adds Sprockets support for Rails 8 applications.
Fixes #36.

lib/hotwire/spark/engine.rb

@@ -18,6 +18,10 @@ class Engine < ::Rails::Engine
       end
     end
 
+    initializer "hotwire_spark.assets" do |application|
+      application.config.assets.precompile << "hotwire_spark.js"
+    end
+
     initializer "hotwire_spark.install" do |application|
       Hotwire::Spark.install_into application if Hotwire::Spark.enabled?
     end