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.

Inizia esaminando il flusso di generazione di `ng g library` e il modo in cui la configurazione di ng-packager controlla gli output della compilazione delle librerie. Il lavoro è completato quando le librerie generate possono compilare Angular Elements/Web Components insieme ai formati ES2015, ES5, UMD e agli altri formati esistenti.

Scritto dal modello di indicizzazione a partire dal testo della issue.