DEV: Fix Ember version lookup (#18136)

The source-of-truth for our ember version is now the installed node_module. The `ember_source` gem carries an old version of Ember and so the constant is no longer useful. We'll be dropping the gem soon.

lib/ember_cli.rb

@@ -49,4 +49,11 @@ module EmberCli
   def self.is_ember_cli_asset?(name)
     assets.include?(name) || name.start_with?("chunk.")
   end
+
+  def self.ember_version
+    @version ||= begin
+      ember_source_package_raw = File.read("#{Rails.root}/app/assets/javascripts/node_modules/ember-source/package.json")
+      JSON.parse(ember_source_package_raw)["version"]
+    end
+  end
 end