Add way to write out default index.html

[TimJentzsch]

Currently it's only possible see the contents of the default index.html via bevy build web --bundle, but it's not that discoverable and also requires you to build the entire app.

Instead, we could do something similar as rustfmt, like rustfmt --print-config default rustfmt.toml.

[TimJentzsch]

Maybe this could be part of a bevy setup command, e.g. bevy setup web-assets, which would create a web folder with the default web assets to allow the user to customize them.