chore(metro-config): refactor metro-config for versioned CLI

[EvanBacon]

Why

• Drop the "testing" platform. It's unclear what this is used for and it's seemingly undocumented.
• Drop the unused "native" platform.
• Export MetroConfig for use in metro.config.js files.
• Import metro dependencies from expo/expo when testing rather than from the project. This will make production usage simpler.
• Assert when expo-asset is not installed. This is an artifact from bare workflow.

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[expo-bot]

Hi there! 👋 I'm a bot whose goal is to ensure your contributions meet our guidelines.

I've found some issues in your pull request that should be addressed (click on them for more details) 👇

⚠️ Suggestion: Missing links in changelog entries

I've added some suggestions below, you can just apply and commit them 😉

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