i18n - implement translation file merging

# 🚀 Feature request

### Command
- [x] build
- [x] serve
- [x] test
- [x] e2e

### Description
Implement a feature to load up multiple specified translation files for a locale and merge the messages when generating translated copies of the project.
This was implemented in the standalone `localize-translate` tool in `@angular/localize` in https://github.com/angular/angular/pull/36792/