Set react as peerDependencies instead of dependencies in package.json

[erikmoren]

From this medium article:

Expressing this as a peerDependency causes npm to not include React in the published package, which reduces package size and avoids the disaster of having multiple versions of React in the user’s target project.

For example:

  "peerDependencies": {
    "react": "^16.3.1",
    "react-dom": "^16.3.1"
  },

Polaris does the same.