timestamp should be replaced with version in service workers

[Rich-Harris]

Describe the problem

#3412 added a version export to $app/env, which defaults to the timestamp at the time of builld. The $service-worker module doesn't expose it — it instead exposes a different timestamp.

Describe the proposed solution

Add version to $service-worker, and replace timestamp with an object that will throw a noisy error if you try to stringify it (which will cover the common case where it's used as a cache key).

Alternatives considered

No response

Importance

nice to have

Additional Information

No response