fix: stop using deprecated serviceWorkerVersion

[alestiago]

Description
Using flutter_web_preloader on latest Flutter versions shows Warnings.

Warning: In index.html:54: Local variable for "serviceWorkerVersion" is
deprecated. Use "{{flutter_service_worker_version}}" template token instead. See
https://docs.flutter.dev/platform-integration/web/initialization for more
details.
Warning: In index.html:134: "FlutterLoader.loadEntrypoint" is deprecated. Use
"FlutterLoader.load" instead. See
https://docs.flutter.dev/platform-integration/web/initialization for more
details.

Steps To Reproduce

1. Create a Flutter web application:

flutter create my_app --platforms web

2. Add flutter_web_preloader:

dart pub global activate mason_cli
mason init
mason add flutter_web_preloader
mason make flutter_web_preloader

3. Run the Flutter application:

flutter run -d chrome

See warning:

Warning: In index.html:54: Local variable for "serviceWorkerVersion" is
deprecated. Use "{{flutter_service_worker_version}}" template token instead. See
https://docs.flutter.dev/platform-integration/web/initialization for more
details.
Warning: In index.html:134: "FlutterLoader.loadEntrypoint" is deprecated. Use
"FlutterLoader.load" instead. See
https://docs.flutter.dev/platform-integration/web/initialization for more
details.

Expected Behavior
No warnings is shown.

Additional Context

• https://docs.flutter.dev/platform-integration/web/initialization
• Related to chore: Confirm Flutter 3.24.0 and Dart 3.5 Support #25

[alestiago]

Snippet of batch loading `flutter_boostrap.js`

{{flutter_js}}
{{flutter_build_config}}

const progressBar = document.querySelector('#progress-bar');
const progressIndicator = document.querySelector('#progress-indicator');

async function readAssets() {
  // NOTE: AssetManifest.json will be deprecated in favour of AssetManifest.bin:
  // https://github.com/VeryGoodOpenSource/flutter_web_preloader/issues/28
  const response = await fetch('assets/AssetManifest.json');
  const manifest = await response.json();
  const assets = Object.values(manifest)
        .map((list) => list.map((url) => 'assets/' + url))
        .reduce((arr, curr) => [...arr, ...curr], []);
  return assets;
}

async function beginPreloading() {
  const assets = await readAssets();

  let totalAssets = assets.length;
  if (totalAssets === 0) {
    // No assets to load, so we can skip the loading process entirely.
    return;
  }

  let loadedAssets = 0;
  const batchSize = 1;

  async function reportProgress() {
    loadedAssets++;

    const value = Math.floor((loadedAssets / totalAssets) * 100) + '%';
    progressIndicator.style.width = value;

    // Artificial delay to make the progress bar more noticeable.
    await new Promise((resolve) => setTimeout(resolve, 500));
  }

  async function load(url) {
    return new Promise((resolve, reject) => {
        const req = new XMLHttpRequest();

        req.onload = function() {
            if (req.status >= 200 && req.status < 300) {
                resolve(req.response);
            } else {
                reject(new Error(`Failed to load: ${req.status} ${req.statusText}`));
            }
        };

        req.onerror = function() {
            reject(new Error('Network error'));
        };

        req.open('GET', url);
        req.send();
    });
  }

  async function loadBatch(urls) {
    const loadPromises = urls.map(url => load(url).then(async () => await reportProgress()));
    try {
        return await Promise.all(loadPromises);
    } catch (error) {
        console.error('Error loading one or more asset:', error);
    }
  }

  for (let i = 0; i < assets.length; i += batchSize) {
    const batch = assets.slice(i, i + batchSize);
    await loadBatch(batch);
  }
}

_flutter.loader.load({
  serviceWorkerSettings: {
    serviceWorkerVersion: {{flutter_service_worker_version}},
  },
  onEntrypointLoaded: async function(engineInitializer) {
    await beginPreloading().finally(() => {
      progressBar.remove();
    });
    const appRunner = await engineInitializer.initializeEngine();

    await appRunner.runApp();
  }
});