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.

まず `ng g library` の生成フローと、ng-packager の設定がライブラリのコンパイル出力をどのように制御しているかを確認します。生成されたライブラリが、既存の ES2015、ES5、UMD などの形式とともに Angular Elements/Web Components をビルドできるようになれば、作業は完了です。

索引モデルが issue の本文から書いたものです。