Move fragment code into separate module

[matejdro]

Currently whetstone always includes fragment dependency.

If used in 100% Compose apps, this dependency is unnecessary and generates unnecessary bloat inside our apps.

It would be nice if all fragment-related code would be moved to a separate module, similarly to the whetstone-compose.

I'm prepared to make a PR for this, if desired. This is potentially breaking, but we could make fragment support opt-out to ensure it does not break anything for the existing users.