text: Title: [google_sign_in_web] Move js-interop layer out of `generated`
  Body: The js-interop layer of the web version of Google Sign In used to be generated by the [js_facade_gen script](https://github.com/dart-lang/js_facade_gen), but it is no longer.
  
  ## Proposal
  
  Move the js-interop layer ([`gapi.dart`, `gapiauth2.dart`...](https://github.com/flutter/plugins/tree/main/packages/google_sign_in/google_sign_in_web/lib/src/generated)) outside of `generated` (or rename `generated` to `js_interop` or similar), so the files don't receive any special treatment.
  
  Also: fix tests, and whatever issues the analyzer may start having with the change, of course.