Export getDefaultConfig for custom configurations

[Salakar]

Summary:

This PR addresses the first issue described in this comment.

Example usage before CLI was moved:
See https://github.com/ueno-llc/react-native-starter/blob/master/metro.config.js

Example usage after move + this PR:

// metro.config.js
const { resolve } = require('path');
const { mergeConfig } = require('metro-config');
const { getDefaultConfig } = require('@react-native-community/cli/build/util/loadMetroConfig');

const root = __dirname;
const reactNativePath = resolve(root, './node_modules/react-native');

const config = {
  resolver: {
    sourceExts: ['js', 'json', 'ts', 'tsx', 'css', 'scss'],
  },
  transformer: {
    babelTransformerPath: require.resolve('./scripts/transformer.js'),
  },
};

module.exports = mergeConfig(getDefaultConfig({ root, reactNativePath }), config);

Test Plan: