feat: add type comment in metro.config.js template (#21898)

# Why

- Add autocomplete to the Metro config object (MetroConfig type is new
in 49).
- Update template to split out config object and

feat: add type comment in metro.config.js template (#21898)

# Why

- Add autocomplete to the Metro config object (MetroConfig type is new
in 49).
- Update template to split out config object and use type comment.
- Export types for `expo/metro-config`.

<!--
Please describe the motivation for this PR, and link to relevant GitHub
issues, forums posts, or feature requests.
-->

# How

<!--
How did you build this feature or fix this bug and why?
-->

# Test Plan

<!--
Please describe how you tested this change and how a reviewer could
reproduce your test, especially if this PR does not include automated
tests! If possible, please also provide terminal output and/or
screenshots demonstrating your test/reproduction.
-->

# Checklist

<!--
Please check the appropriate items below if they apply to your diff.
This is required for changes to Expo modules.
-->

- [ ] Documentation is up to date to reflect these changes (eg:
https://docs.expo.dev and README.md).
- [ ] Conforms with the [Documentation Writing Style
Guide](https://github.com/expo/expo/blob/main/guides/Expo%20Documentation%20Writing%20Style%20Guide.md)
- [ ] This diff will work correctly for `expo prebuild` & EAS Build (eg:
updated a module plugin).

---------

Co-authored-by: Expo Bot <[email protected]>

show more ...