Build out app cache sync event

[DanielBaulig]

Currently the app cache will get updated when the user loads the application over the network. If the network is not reachable while the app gets loaded, the app cache will not get updated for the session. I could use the Background Sync API to trigger an update of the app cache as soon as the network recovers during the same session.

[DanielBaulig]

Some more things to consider for app cache:

• drop manifest plugin and use vite-manifest to query for changes
• consider switching to network first for entry documents (i.e. index.html). Page load will be slower, but user would always get the most recent version of the app immediately.
• only start refreshing/populating app-cache after page has fully loaded and rendered, i.e. document informs the SW that it can go an update the app cache
• build out a UI in the app that informs the user that a new version of the app is available and prompts them to reload. This could be used for both a post load new app version and if the app version gets refreshed upon network recovery if loaded from SW.