Use env vars to customize build

[kendallroth]

Description

Support using environment variables to customize the app's behaviour, as documented by Expo. Environment variables must be set in various locations depending on the workflow: development must run as part of expo npm command, builds must be present in appropriate channels in eas.json, and updates must be present as env variables. Variables can be accessed either in app.config.ts or from the expo-constants library (can use optional env.config.ts file wrapper).

Changes

• Pass environment variables to builds in eas.json
• Investigate passing environment variables in development
• Access environment variables in env.config.ts (with defaults)
• Investigate and document how to support env vars in updates