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

**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