Restore base config merge in metro-config (#38092)

Summary:
Pull Request resolved: #38092

Reverts #36777.

This is motivated by reducing user friction when the widespread assumption is for `react-native/metro-config` to export a complete Metro config, as with Expo/rnx-kit, and like previously understood uses of `metro-config`. See facebook/metro#1010 (comment) for further notes.

Fixes:
- facebook/metro#1010
- #38069
- kristerkari/react-native-svg-transformer#276

Note that we do not intend for `react-native/metro-config` to directly export `assetExts` etc — these can be accessed on the `resolver` property from the full config object.

Changelog: [General][Changed] `react-native/metro-config` now includes all base config values from `metro-config`

Reviewed By: robhogan

Differential Revision: D47055973

fbshipit-source-id: 5ad23cc9700397110de5c0e81c7d76299761ef0a

packages/metro-config/index.js

@@ -8,9 +8,9 @@
  * @noformat
  */
 
-/*:: import type {MetroConfig} from 'metro-config'; */
+/*:: import type {ConfigT} from 'metro-config'; */
 
-const {mergeConfig} = require('metro-config');
+const {getDefaultConfig: getBaseConfig, mergeConfig} = require('metro-config');
 
 const INTERNAL_CALLSITES_REGEX = new RegExp(
   [
@@ -37,8 +37,8 @@ const INTERNAL_CALLSITES_REGEX = new RegExp(
  */
 function getDefaultConfig(
   projectRoot /*: string */
-) /*: MetroConfig */ {
-  return {
+) /*: ConfigT */ {
+  const config = {
     resolver: {
       resolverMainFields: ['react-native', 'browser', 'main'],
       platforms: ['android', 'ios'],
@@ -76,6 +76,11 @@ function getDefaultConfig(
     },
     watchFolders: [],
   };
+
+  return mergeConfig(
+    getBaseConfig.getDefaultValues(projectRoot),
+    config,
+  );
 }
 
 module.exports = {getDefaultConfig, mergeConfig};