Fix react-native-macos-init to not break react-native-windows-init

[tom-un]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

When react-native-windows-init modifies a project, it adds a react-native.config.js file. When react-native-macos-init modifies a project, it also adds a react-native.config.js file wiping out any previous contents.

This change make it such that the macOS requirements are appended to an existing react-native.config.js if one already exists, or creates a new one.

react-native-windows-init creates a config with the contents:

const fs = require('fs');
const path = require('path');
module.exports = {
  reactNativePath:  fs.realpathSync(path.resolve(require.resolve('react-native-windows/package.json'), '..')),
};

react-native-macos-init will now append:

const macSwitch = '--use-react-native-macos';
if (process.argv.includes(macSwitch)) {
  process.argv = process.argv.filter(arg => arg !== macSwitch);
  process.argv.push('--config=metro.config.macos.js');
  module.exports = {
    reactNativePath: 'node_modules/react-native-macos',
  };
}

The react-native-windows-init package will need a similar change to prepend its required contents to react-native.config.js. When it does this, then react-native-macos-init can be run before react-native-windows-init and vis versa.

Note: The need for react-native.config.js at all in either windows or macOS projects is temporary: when both projects are at react-native 0.62 and react-native-community/cli#1115 lands, then both projects can use npmPackageName instead.

Focus areas to test

The script was tested against project folder with and without pre-existing react-native.config.js files.

Microsoft Reviewers: Open in CodeFlow

[pull-bot]

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