Jump to next/previous prompt in haskell-interactive-mode

New functions:

- `haskell-interactive-mode-next-prompt`
- `haskell-interactive-mode-prev-prompt`

New keybindings in `haskell-interactive-mode-map`:

- `C-c C-n` jump to next prompt
- `C-c C-p` jump to previous prompt

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Best,
Markus