fix(preprocessor): return object, not just code (facebook#33576)

Summary:
Jest 28 will error if only a string is returned from a transfomer, it needs to be an object of `{code: string, map?: object}`. Returning an object has been supported since jestjs/jest#2290, released in Jest v20.

## Changelog

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[Internal] [Fixed] - Return object from Jest preprocessor

Pull Request resolved: facebook#33576

Test Plan: Green CI?

Reviewed By: motiz88

Differential Revision: D35426748

Pulled By: robhogan

fbshipit-source-id: 3edb79b62b2d34274be192061817d3fcd8a7ba17

jest/preprocessor_DO_NOT_USE.js

@@ -30,15 +30,15 @@ babelRegisterOnly([]);
 
 const transformer = require('metro-react-native-babel-transformer');
 module.exports = {
-  process(src /*: string */, file /*: string */) /*: string */ {
+  process(src /*: string */, file /*: string */) /*: {code: string, ...} */ {
     if (nodeFiles.test(file)) {
       // node specific transforms only
       return babelTransformSync(src, {
         filename: file,
         sourceType: 'script',
         ...nodeOptions,
         ast: false,
-      }).code;
+      });
     }
 
     const {ast} = transformer.transform({
@@ -111,7 +111,7 @@ module.exports = {
         sourceMaps: true,
       },
       src,
-    ).code;
+    );
   },
 
   getCacheKey: (createCacheKeyFunction([