launch handling, and adding a "service-worker" client-mode

[mkruisselbrink]

I'd like to continue last years discussion about launch handling for web apps, and especially a possible extension of the existing https://wicg.github.io/web-app-launch/ proposal with a new "service-worker" client mode, currently roughly described at WICG/web-app-launch#93 and https://github.com/WICG/web-app-launch/blob/main/sw_launch_event.md