Port cra.sh development task to javascript #2280

A quick improvement I'd thought I'd propose for those of us looking to contribute to CRA from a Windows machine.

As part of end to end testing while contributing to CRA we can use the npm run create-react-app my-app command to generate a new app. However this fails when run on Windows:

$ npm run create-react-app test

> @ create-react-app C:\Projects\create-react-app
> tasks/cra.sh "test"

'tasks' is not recognized as an internal or external command,
operable program or batch file.

A simple solution may be to port the cra.sh shell script to javascript, which would make it easier to run on a Windows based machine. There are workarounds such as installing Bash on Windows 10, but this would make life a little easier :)

I'd be happy to submit a PR if we think it's worthwhile.