DEV: Ensure Embroider sourcemaps are collected by Sprockets (#23468)

Names of sourcemaps are not necessarily equal to the js file names. Instead, we can check the `sourceMappingURL` comment to find the map's filename.

lib/ember_cli.rb

@@ -59,6 +59,10 @@ module EmberCli
     {}
   end
 
+  def self.parse_source_map_path(file)
+    File.read("#{dist_dir}/assets/#{file}")[%r{^//# sourceMappingURL=(.*)$}, 1]
+  end
+
   def self.is_ember_cli_asset?(name)
     assets.include?(name) || name.start_with?("chunk.")
   end