fix(js): allow pinning importmaps with gem (#49)

Since import maps JSPM and the javascript ecosystem are a mess in their current state, allow import maps to match the assets served by the gem rather than download them from JSPM.

lib/turbo_boost/elements/engine.rb

@@ -11,11 +11,19 @@ def self.config
 
   class Engine < ::Rails::Engine
     isolate_namespace TurboBoost::Elements
+
     config.turbo_boost_elements = ActiveSupport::OrderedOptions.new
+    config.turbo_boost_elements.precompile_assets = true
 
     ActiveSupport.on_load(:action_controller_base) do
       # `self` is ActionController::Base
       helper TurboBoost::Elements::ApplicationHelper
     end
+
+    config.after_initialize do |app|
+      if app.config.respond_to?(:assets) && app.config.turbo_boost_elements.precompile_assets
+        app.config.assets.precompile += %w[@turbo-boost/elements.js]
+      end
+    end
   end
 end