SW-Launch: Consider some kind of static routing and/or filtering

[mkruisselbrink]

Spinning up a service worker for every navigation can be expensive. For the service worker fetch event there is a static routing API to make it possible to skip running the service worker depending on certain routing rules. We should consider if we need something similar for launch events as well. Either some kind of include/exclude pattern in the manifest or a similar route matching API at service worker install time.

[mkruisselbrink]

One option might be to add a third field to the RouterRule dict to override launch handling behavior for any navigation matching the condition, possibly combined with making source optional, defaulting to "fetch-event".