Feature: Add `extends` config loading support

[huntie]

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Description

We'd like Metro's configuration to accept an extends key, similar to other tools such as ESLint and TypeScript.

This would benefit Metro users in enabling a more convenient and concise API that abstracts away the need for mergeConfig() today.

// metro.config.js
module.exports = {
  extends: ['@react-native/metro-config'],
  ...
};

Related context:

• RFC: Add @react-native/metro-config package react-native#36502

Rough design

The implementation for this feature should live in the metro-config package.

• Update the relevant MetroConfig type constituents to add extends?: $ReadOnlyArray<string>.
• Update loadMetroConfigFromDisk to read extends if present.
  • For each value in extends, attempt to require the config path relative to the filepath of the input config file, and apply the imported object in order via mergeConfig().
  • Throw an error if a config path cannot be resolved.
• Add one or more test cases in packages/metro/src/integration_tests/.
• Update documentation to illustrate usage of this feature.

[CaioFernandoDev]

Hey @huntie, I would love to work on this feature. I am new to this repo, but I think I can do this.

[huntie]

Hi @CaioFernandoDev! Awesome, please reach out if you have any questions — either here, or feel free to DM me on Twitter.

[antongolub]

As far as the prev assignment has been removed, I'll try to continue, if no one minds.

@huntie, could you assist with this?

[0xSaksham]

Looks like no one is working on this. Would love to talk about this issue and DM'ed you on X @huntie

[padunk]

Is this still on the roadmap? I'd love to work on this.