RN/Expo Unite!

[frankcalise]

Please verify the following:

• [x] yarn ci:test jest tests pass with new tests, if relevant
• [x] README.md has been updated with your changes, if relevant

Describe your PR

Implements Expo/Vanilla Unification (#1983)

• [x] Update the scripts to have both yarn ios and yarn expo:ios (etc) depending on what you want to run -- running on Expo Go will always be prefixed with yarn expo:*
• [x] Ensure that index.js and App.js load appropriately based on which is run
• [x] Unify any *.expo.* files where appropriate
• [x] Let the user know after running new how to run via vanilla vs Expo
• [x] Updated help, new commands to remove Expo references
• [x] Updated README to remove reference to Expo
• [x] Added warning message if user still supplies --expo

Tested commands

• [x] yarn ios
• [x] yarn android
• [x] yarn expo:start
• [x] yarn expo:web

Other tasks

• [x] Remove Expo warning about .expo.* file extensions
• [ ] Testing with detox
• [x] Run expo doctor for boilerplate\ for clean terminal after igniting an app

Example of warning message: