Bring more to Internal module

The internal module is really empty and doesn't give a lot of room for extending Haskeline, here are a few use cases the library doesn't provide ways to do without forking, like for example:

- extending the editing mode (reusing things from Vi.hs or Emacs.hs)
- using haskeline within an UI ( the haskeline-brick project had to fork haskeline)
- hooking into character changes/ operations and doing IO with them.

Start by reading the Internal module and the existing editing implementations in Vi.hs and Emacs.hs. Review the haskeline-brick fork to understand which extension points it needed. This issue needs a scoped API decision before implementation; done would mean agreed support for the selected editing, UI, or character-operation hooks.

Written by the indexing model from the issue text.