feat(cli, metro-config): environment variable support

[EvanBacon]

Why

Expo supports the standard .env resolution.

• It's nice to be able to use uncommitted values in your app, based on the environment. This feels very familiar to web developers.
• Values that are prefixed with EXPO_PUBLIC_ will be inlined in the bundle when bundling normally (e.g. not for Node.js).
• .env files are loaded into memory and applied to the process during a run. This also means that they're available in app.config.js.
• During development-only, environment variables are exposed on the process.env object (non-enumerable) to ensure they're available between fast refresh updates.

How

• Create new package @expo/env which is used to hydrate env variables in a unified way. I plan to open another PR in eas-cli which uses this package to fill in environment variables before uploading. NOTE: environment variables that are defined in eas.json are not available in Expo CLI when building locally, but are available in the cloud since they'll be on the process, this means they effectively emulate .env.production.
• Update templates to gitignore local env files.
• Add basic documentation to the versioned metro guide (more to come).

Test Plan

• E2E rendering test
• E2E Node.js rendering test
• Unit tests for serializer
• Tests for env package

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for expo prebuild & EAS Build (eg: updated a module plugin).

[amandeepmittal]

Left some suggestions.

[amandeepmittal]

What does "inlined in Node Modules" mean here?