fix(expo-router)!: compile as ESM

[EvanBacon]

Why

We changed the compilation to cjs to support importing manifest code in the CLI without needing to bundle, but as a result, all of react-native-web is pulled in to web projects. This PR changes the bundle format back to esm, and adds a cjs build step to the prepare script which can be used in CLI to access the manifest functions. The change results in decent bundle size savings: 950kb -> 880kb in static-rendering e2e test.

We'll need a follow up PR to do something similar to expo/metro-runtime so we aren't pulling in everything in development too.

Test Plan

• All tests should continue passing.

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 npx expo prebuild & EAS Build (eg: updated a module plugin).