Remove 'import' from default Package Exports conditions (#955)

Summary:
X-link: facebook/metro#955

Pull Request resolved: #36584

Changelog:
[General][Changed] - Default condition set for experimental Package Exports is now `['require', 'react-native']`

The [Exports RFC](https://github.com/react-native-community/discussions-and-proposals/blob/main/proposals/0534-metro-package-exports-support.md) had assumed that supporting the `"import"` condition was a syntax-only difference, given we are not in a Node.js environment — and so was worthwhile to support for maximal ecosystem compatibility.

{F915841105}

This assumption is similar to [`--moduleResolution bundler` in TypeScript 5.0](microsoft/TypeScript#51669):

> bundlers and runtimes that include a range of Node-like resolution features and ESM syntax, but do not enforce the strict resolution rules that accompany ES modules in Node or in the browser
> -- microsoft/TypeScript#51669 (comment)

However, robhogan has rightly pointed out that **we should not do this!**

- ESM (once transpiled) is **not** simply a stricter subset of in-scope features supported by CJS. For example, it supports top-level async, which would be breaking at runtime.
- We recently made the same change for our Jest environment:
    - 681d7f8

As such, we are erring on the side of correctness and supporting only `['require', 'react-native']` in our defaults. At runtime, all code run by React Native is anticipated to be CommonJS. `"exports"` will instead allow React Native to correctly select the CommonJS versions of modules from all npm packages.

Metro changelog: [Experimental] Package Exports `unstable_conditionNames` now defaults to `['require']`

Reviewed By: robhogan

Differential Revision: D44303559

fbshipit-source-id: 0077e547e7775e53d1e4e9c3a9d01347f4fb7d4a

packages/metro-config/index.js

@@ -42,7 +42,7 @@ function getDefaultConfig(
     resolver: {
       resolverMainFields: ['react-native', 'browser', 'main'],
       platforms: ['android', 'ios'],
-      unstable_conditionNames: ['import', 'require', 'react-native'],
+      unstable_conditionNames: ['require', 'react-native'],
     },
     serializer: {
       getPolyfills: () => require('@react-native/js-polyfills')(),