[FEATURE REQUEST] Inline preview window

[JE-Amouel]

Checklist

• I have read through the manual page (man fzf)
• I have searched through the existing issues
• For bug reports, I have checked if the bug is reproducible in the latest version of fzf

Output of fzf --version

futur

OS

• Linux
• macOS
• Windows
• Etc.

Shell

• bash
• zsh
• fish

Problem / Steps to reproduce

HI,
I'm a fan of all the progress recently made by the fzf team, thank you

If you allow me, I would like to submit a functionality suggestion,

I have been following the progress of fzf for several months, and there is a feature that I would like to have in fzf,

the inline preview window, the current fzf preview window can only be fixed either vertically or horizontally to the selector
I would really appreciate having an online preview window, which is dynamic and scalable,
ie,
that still in the selector I could have the preview directly either, under the selector or above the selector,

but also if the window is scalable I could have the equivalent of a floating window next to my selector

here is a montage that a make with krita and online gif editor imgflip.com
here we suppose a compose: fzf --preview "cat {}" --preview-window = inline,down,40%
where inline would be the new latyout option.

quick overview;

as you may see, preview window is displayed at the bottom of the currently focused selection.

THANKS 🙏