Spawn and Manage Child Processes and Node.js Streams with execa

CLI's gain an extra level of power when you realize they can start and manage other CLI's and run multiple concurrent child processes (great for farming out work on multi-core machines!). You can even use it to improve developer experience in really small ways. For example, create-react-app famously scaffolds out a React app for you, and also installs the relevant node_modules. We'll learn how to do this two ways: with execa and with a dedicated yarn-or-npm utility, because this is such a common usecase.

But before all that - we must learn and be comfortable with the raw Node.js child_process and I/O Stream API's. So that's where we begin.