DEV: Ensure service-worker sourcemap URLs do not duplicate extension (#16704)

`script_asset_path('.../blah.js.map')` was appending `.js`, which would result in a filename like `.js.map.js`. It would also lose the `/assets` prefix, since the map files are not included in the sprockets manifest.

This commit updates the sourceMappingURL rewriting logic to calculate the service-worker's own JS url, and then append `.map`.

spec/requests/static_controller_spec.rb

@@ -420,7 +420,7 @@ describe StaticController do
       get "/service-worker.js"
       expect(response.status).to eq(200)
       expect(response.content_type).to start_with("application/javascript")
-      expect(response.body).to include("sourceMappingURL=/assets/service-worker-abcde.js.map")
+      expect(response.body).to include("sourceMappingURL=/assets/service-worker.js.map\n")
     end
   end
 end