Refactor CoreComposableResolver into a service

Problem/Motivation

Currently CoreComposableResolver is implemented as a basic class with static methods. This means the class is hardcoded and cannot be overridden in client projects without having to patch the module.

The class is also not following best practices. For example it doesn't use dependency injection, doesn't have an interface, and uses private methods which limit extensibility.

Steps to reproduce

Try to override anything in CoreComposableResolver, for example the way language fallbacks are handled.

Proposed resolution

1. Transform the class into a service.
2. Add an interface.
3. Make all private methods protected.
4. Inject dependencies.

Remaining tasks

This will be a B/C break for existing projects that are calling in to the CoreComposableResolver class. We should decide on how to implement a B/C layer.

API changes

CoreComposableResolver will become a service.

Data model changes