Allow schematics to create file with permissions

### Bug Report or Feature Request (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [x] feature request
```

### Versions

any

### Desired functionality

I'm writing schematics to help colleagues scaffold applications and libraries following our internal guidelines. One part of our set-up is the use of a file called `Taskfile` to contain scripts instead of cramming complicated one-liners in yarn run-scripts. This file should be executable, but a schematic cannot create executable files. The result is a broken package because `./Taskfile: Permission denied`.

I've got a simple workaround:

The schematic in question is based on `@schematics/angular:library`. It runs the `NodePackageInstallTask` at the end. This allows me to create a selfdestructing `preinstall` script in the newly scaffolded `package.json`:

```json
{
"//": "...",
"scripts": {
"preinstall": "chmod +x Taskfile && ex +g/preinstall/d -cwq package.json",
"prepublish": "./Taskfile build",
"test": "./Taskfile test"
}
}
```

This works, but it feels like a hack to me. It would be cleaner if either

- file permissions were respected, as the source file for the scaffold has correct permissions; or
- the schematic could modify permissions

Commencez par le point d’entrée @schematics/angular:library et suivez la manière dont il crée des fichiers avant l’exécution de NodePackageInstallTask. Utilisez la Taskfile exécutable et le contournement généré pour package.json comme cas de reproduction. Le travail est terminé lorsqu’un schematic peut créer ou modifier un fichier avec des permissions d’exécution sans dépendre du script de preinstall qui s’autodétruit.

Rédigé par le modèle d'indexation à partir du texte de l'issue.