feat: make the current cmdline editable in place

[IndianBoy42]

Sometimes I start typing a long complicated command and then I get annoyed with the lack of vim keybindings. My shell (fish) has a keybinding to open the current commandline in $EDITOR but then I lose sight of the scrollback, so its suboptimal.

kitty-scrollback.nvim has the potential to be useful here. Currently adding to the current commandline is easy, but there is no easy way to select, edit or replace the existing commandline.