Support web workers in libraries

# 🚀 Feature request

### Command (mark with an `x`)

```
- [ ] new
- [ ] build
- [ ] serve
- [ ] test
- [ ] e2e
- [x] generate
- [ ] add
- [ ] update
- [ ] lint
- [ ] xi18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc
```

### Description
I would like to use Web Workers in an Angular 8 library project.
In version 8.1.1 this seems to be supported for project type 'application' only:
```console
ng generate webWorker my-module/myWorker --project my-lib
Web Worker requires a project type of "application".
```

### Describe the solution you'd like
Generating a Web Worker should work for project type 'library' as well.

### Describe alternatives you've considered
Rewrite TypeScript worker code to JavaScript file.

まず `ng generate webWorker my-module/myWorker --project my-lib` を実行し、generate コマンドのプロジェクトタイプ検証を追跡します。コマンドが、worker にはアプリケーションが必要だと報告する代わりに、ライブラリプロジェクト用の Web Worker を生成すれば完了です。

索引モデルが issue の本文から書いたものです。