[Yarn] Support for copy-pasting packages

It would be great if we could go from the message:
```
npm install ${notInstalled.map((pkg) => atLatest(pkg)).join(' ')} --save-dev
```
to 
```
yarn add -D ${notInstalled.map((pkg) => atLatest(pkg)).join(' ')}
```
If a `yarn.lock` file exists in the projects root directory.