Add progress indicator for initial load of web app

[TimJentzsch]

The Wasm binary of a Bevy app can be quite big, especially in debug mode. This can result in significant loading times for the website.

We should check if we can add a simple progress indicator to the loading to improve the user feedback.

The server might provide a content-length header indicating the total size of the binary, which we can then use to calculate the progress.

As we only control the dev server and not the deployed version, the implementation should gracefully handle if the server doesn't provide the required data.

[TimJentzsch]

This might actually be quite complex. We download the binary with async import right noe, which AFAIK doesn't provide any callbacks for reporting progress.

Even with fetch it's complicated, because content-length will be the size of the compressed data (if e.g. gzip is used), but on the client we can only track the downloaded uncompressed data https://stackoverflow.com/a/64123890