Always output an 'id' field in the manifest, or, force users to specify an 'id'

[dmurph]

Is your feature request related to a problem? Please describe.
We have one user who has a PWA which has had it's id change, as the start_url was changed. This is bad, as it means they made a different app. In the web platform world. this is like changing the android package id.

Describe the solution you'd like
Users of bubblewrap should be protected from this issue, either by being forced to specify an 'id', or by having bubblewrap specify one using existing metadata that is guaranteed to be stable.

https://developer.chrome.com/docs/capabilities/pwa-manifest-id
https://docs.google.com/presentation/d/1QqriRwxqzo8Dqwj3BOixE6AWIm0Kr4y9-ScOm-MKhMs/edit#slide=id.g2f9677cb0f8_0_0