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

@schematics/angular:library エントリポイントから開始し、NodePackageInstallTask が実行される前にどのようにファイルを作成するかを追ってください。実行可能な Taskfile と生成された package.json の回避策を再現ケースとして使用してください。Schematic が自己破壊する preinstall スクリプトに依存せずに、実行権限付きのファイルを作成または変更できれば完了です。

索引モデルが issue の本文から書いたものです。