Add option to open 'ChatGPTRun explain_code' in split

Currently, `ChatGPTRun explain_code` opens the answer in floating window hiding the explained code, which is not continent and not preservable in case one wants to keep the answer for later use.
Please add support to open the answer in split.
Or maybe there's existing nvim way to convert floating win to split? Couldn't find one...

Start at the ChatGPTRun explain_code entry point and trace how its answer is opened in a floating window. Add a split-window option while preserving the existing behavior, then verify that explain_code can display its answer beside the explained code.

Written by the indexing model from the issue text.