feat(cli, router, babel): add router root to metro cache

[EvanBacon]

Why

• Add support for invalidating the cache when the app directory changes. We use this feature in our E2E tests so it would be good to ensure it works properly.
• This change also enables us to move the Expo config read operation out of the babel preset which should generally be safer and faster.
• By adding this invalidation we also ensure that the cache is cleared when expo-router is installed, this should help reduce issues related to manual expo-router installation in existing projects.
• This change also deletes expo-router/babel in favor of babel-preset-expo which is properly tested.
• I've also added the option to change the root directory to the docs and noted that it's a dangerous operation that may lead to issue closing.

Test Plan

• Ensure project continue working.
• Ensure the cache clears when the root directory changes.

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).

[amandeepmittal]

Few small suggestions but LGTM!

[pvinis]

Seems like on 2.0.14 we still get

ValidationError: Invalid options object. expo-router config plugin has been initialized using an options object that does not match the API schema.
 - options has an unknown property 'root'. These properties are valid:
   object { origin?, headOrigin?, unstable_src?, asyncRoutes? }

when trying to use "root" like the docs here https://docs.expo.dev/router/reference/src-directory/#custom-directory.