Allow referencing (parts of) ServiceWorker manifest from client

[jarrodldavis]

The Cache API is available to the main document context via window.caches. It would be nice to be able to get the timestamp from @sapper/service-worker so that, for example, I could clear the offline cache when signing a user out.

Currently, importing @sapper/service-worker in a component (or a helper module imported by a component) either causes a rebuild loop (in dev mode) or a non-working require('@sapper/app') in the server's build output (with Rollup).