imp: Add titlePlaceholder in template.config.js (#26218)

Summary:
In the latest `react-native-cli` we've added more flexibility for setting application title. New config key was introduced - `titlePlaceholder` - which will be changed during init process.

PR: https://github.com/react-native-community/cli/pull/650

## Changelog

[General] [Added] - Introduce `titlePlaceholder` for template configuration.
Pull Request resolved: https://github.com/facebook/react-native/pull/26218

Test Plan: Initialization works with the default template

Differential Revision: D17091492

Pulled By: shergin

fbshipit-source-id: 239110f2ad6f817d750575fa366ab49d146b36c1

template.config.js

@@ -7,5 +7,6 @@
 
 module.exports = {
   placeholderName: 'HelloWorld',
+  titlePlaceholder: 'Hello App Display Name',
   templateDir: './template',
 };