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.

首先檢視 `ng g library` 的產生流程，以及 ng-packager 設定如何控制程式庫的編譯輸出。當產生的程式庫能夠在現有的 ES2015、ES5、UMD 和其他格式之外，同時建置 Angular Elements/Web Components 時，這項工作就完成了。

由索引模型根據 Issue 內容生成。