Angular integration installs wrong version of core-js

Steps to reproduce:
```
rails new . --webpack=angular --force
bundle exec rails assets:precompile
...
ModuleNotFoundError: Module not found: Error: Can't resolve 'core-js/es6/reflect' in 'app/javascript/hello_angular'
...
```

Forcing version 2 of `core-js` fixes things:
```
yarn add core-js@~2
bundle exec rails assets:clobber
bundle exec rails assets:precompile
```