Setup expo app & yarn workspaces

[Charly6596]

The idea is to build the following folder structure:

kanamoji
├── packages
│   ├── app # phone app
│   ├── controllers # shared code package, referenced by app and web packages
│   └── web
├── package.json # workspace package
└── README.md

The controllers package will contain shared platform-agnostic code, used by the app and web packages. The rest of the code will be in the package corresponding to its platform, but we still should write it as agnostic as possible.