Restore 'import' in default Package Exports conditions (facebook#36902)

Summary:
Pull Request resolved: facebook#36902

Reverts facebook#36584.

Since we've come across example packages (typically targeting Node.js) which only distribute ESM, we believe it's more helpful to return to asserting the `"import"` condition by default, for maximum compatibility. The above issue and comments outline the pros/cons.

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

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

Reviewed By: robhogan

Differential Revision: D44962143

fbshipit-source-id: d2004df313c858c70bc41c5e096ae2ab9f627bc1

packages/metro-config/index.js

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