Adding an example app to bugsnag-expo

[hannah-smartbear]

• Adding an expo example app to show how to use Bugsnag with an expo project
• Created the example app using npx create-expo-app
• It has 3 buttons (to trigger a handled error, an unhandled error, and a render error)
• I have checked that it runs as expected on both android and iOS
• I also added a README similar to those in other example apps