Update new project template to metro-react-native-babel-preset

[janicduplessis]

The babel plugin was recently moved to the metro repo and published under metro-react-native-babel-preset. We need to make sure to update all references to the old babel plugin as it will no longer be updated. I think the main thing that needs updating is the new project template .babelrc (https://github.com/facebook/react-native/tree/master/local-cli/templates/HelloWorld) and package.json (https://github.com/facebook/react-native/blob/master/local-cli/init/init.js#L102).

Note that it it needs to be set as module:metro-react-native-babel-preset in .babelrc since it does not respect the standard babel preset naming (maybe we could fix that also).

We also need to communicate this properly in the release notes for existing apps.

[jamsch]

So this means that metro-react-native-babel-preset is going to be a drop in replacement for babel-preset-react-native@5.x / Babel 7 RN projects?

[janicduplessis]

@jamsch Yes it is exactly the same, it was just moved to the metro repo and published under acnew name.

[kelset]

Yes, I guess this needs to be done before 0.57.0 is released (the first RC will be cut today).

As a side-change to this it would be useful to migrate the .babelrc config to the new babel.config.js since, quoting from the docs:

If Babel finds other .babelrc files while transpiling files in subfolder, it will merge the configuration together.
Note that it can lead to undefined behaviors, and we recommend to use babel.config.js documentation instead.

[grabbou]

Let's check this issue once we release RC, we can test the full workflow and see where the references to old babel plugin are and how that affects the project itself.

[janicduplessis]

@grabbou It should still work with the old plugin as long as there are no significant changes to it. But yeah, we should try to update it before the next release just in case.

[rafaellincoln]

@kelset @grabbou Has not this been done yet?

[kelset]

It has been, via this #20653 (linked above)