Fix helloworld Jest preset after react-native/jest-preset subpath removal

Summary:
#57481 removed the `react-native/jest-preset` subpath redirect, but the
in-repo `private/helloworld` template still referenced `preset: 'react-native'`,
which Jest resolves to the now-deleted `react-native/jest-preset.js`. This broke
all `test_ios_helloworld*` CI jobs on 0.87-stable at the `yarn test` step:

  Validation Error: Module react-native should have "jest-preset.js" or
  "jest-preset.json" file at the root.

Since `private/helloworld` is excluded from the yarn workspace and installed
standalone, it also needs `@react-native/jest-preset` as an explicit
devDependency for the preset to resolve.

- Point jest.config.js preset at `@react-native/jest-preset`.
- Add `@react-native/jest-preset` to devDependencies.

Changelog: [Internal]

Test Plan:
CI: `test_ios_helloworld*` jobs pass (previously failing at `yarn test`).

Author: zeyap

private/helloworld/jest.config.js

@@ -9,5 +9,5 @@
  */
 
 module.exports = {
-  preset: 'react-native',
+  preset: '@react-native/jest-preset',
 };

private/helloworld/package.json

@@ -22,6 +22,7 @@
     "@react-native/babel-preset": "0.87.0-rc.0",
     "@react-native/core-cli-utils": "*",
     "@react-native/eslint-config": "0.87.0-rc.0",
+    "@react-native/jest-preset": "0.87.0-rc.0",
     "@react-native/metro-config": "0.87.0-rc.0",
     "@react-native/typescript-config": "0.87.0-rc.0",
     "@types/jest": "^29.5.14",