Bundle multiple `prerender_pages_unbundled()`

[dgp1130]

I've used multiple prerender_pages() targets to build different pages independently and merge them together with web_resources(). However this isn't ideal, because each prerender_pages() target bundles all its JavaScript independently. This means any shared chunks between multiple prerender_pages() targets won't actually be shared at runtime.

prerender_pages_unbundled() skips the work which should be skipped, but I don't have a good way to run that work later. Maybe we should have a bundle_site() rule which takes multiple prerender_pages_unbundled(), bundles the JavaScript and injects the resources.