Implement autolinking for iOS

[elisechant]

Implement autolinking so users don't need to run react native link for this dependency.

From RN CLI:

$ react-native run-ios
error React Native CLI uses autolinking for native dependencies, but the following modules are linked manually:
  - react-native-version-number (to unlink run: "react-native unlink react-native-version-number")
This is likely happening when upgrading React Native from below 0.60 to 0.60 or above. Going forward, you can unlink this dependency via "react-native unlink <dependency>" and it will be included in your app automatically. If a library isn't compatible with autolinking, disregard this message and notify the library maintainers.
Read more about autolinking: https://github.com/react-native-community/cli/blob/master/docs/autolinking.md