Add Angular Elements to the "ng g library" compilation's results

# 🚀 Feature request

### Command (mark with an `x`)
```
- [x] generate
```

### Description
We know that the "ng g library" helps us with the creation of reusable libraries. The components we create into ous libs can be compiled into multiple JS formats (ES2015, ES5, UMD, ...), which is awesome! Thank you CLI team! However, would it be possible to have them compiled into Web Components as well?

### Describe the solution you'd like
I know that Angular Elements assist with the creation of Web Components. However, I was not able to add Elements to the libs created to "ng g library". Nonetheless, rather than adding it manually... it would be amazing if the CLI team could add that capability to the "ng g library" directly... so that this cmd could insert the necessary configuration for the ng-packager to use and automatically build Web Components alongside the ES2105, UMD, and other formats.

### Describe alternatives you've considered
None yet.

Beginne mit der Überprüfung des Generierungsablaufs von `ng g library` und dessen, wie die ng-packager-Konfiguration die Ausgaben der Bibliothekskompilierung steuert. Die Arbeit ist abgeschlossen, wenn generierte Bibliotheken zusätzlich zu den bestehenden ES2015-, ES5-, UMD- und anderen Formaten Angular Elements/Web Components erstellen können.

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