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.

首先检查 `ng g library` 的生成流程，以及 ng-packager 配置如何控制库的编译输出。当生成的库能够在现有的 ES2015、ES5、UMD 和其他格式之外，同时构建 Angular Elements/Web Components 时，这项工作就完成了。

由索引模型根据 Issue 内容生成。