feat: Add i18n support in CustomElement template formatting

Summary of changes
- Enhanced `CustomElement` class with i18n property for template formatting. The property is initialized to false and can be enabled for internationalization support.
- Implemented `substituteI18n` function to handle i18n formatting for HTML content using a `Formatter` instance, which applies labels to the templates.
- Updated the `initHtmlContent` and `initShadowRoot` methods to call `substituteI18n`, thereby applying i18n formatting when rendering HTML.

- Updated the import sequence in the JSDOM tests to ensure consistent element polyfills are loaded before tests run.
- Added debug logging for key setting in the JSDOM initialization to assist in debugging any issues with key assignments from the `window` object.

These changes allow developers to specify an i18n option when using the `CustomElement`, enhancing localization capabilities for template rendering. The adjustments in the tests enhance reliability and traceability during the testing process.