(Experimental) Improve Sixel graphics support (#2544)

Progress:

* Sixel image can now be displayed with other text, and is scrollable
* If an image can't be displayed entirely due to the scroll offset, fzf
  will render a wireframe to indicate that an image should be displayed
* Renamed $FZF_PREVIEW_{WIDTH,HEIGHT} to $FZF_PREVIEW_PIXEL_{WIDTH,HEIGHT}
  for clarity
* Added bin/fzf-preview.sh script to demonstrate how to display an image
  using Kitty or Sixel protocol

An example:

  ls *.jpg | fzf --preview='seq $((FZF_PREVIEW_LINES*9/10)); fzf-preview.sh {}; seq 100'

A known issue:

* If you reduce the size of the preview window, the image may extend
  beyond the preview window

CHANGELOG.md

@@ -3,27 +3,21 @@ CHANGELOG
 
 0.43.1
 ------
-- (Experimental) Added support for Sixel graphics in the preview window
-  ```sh
-  # 1. $FZF_PREVIEW_WIDTH and $FZF_PREVIEW_HEIGHT will be set to the pixel width
-  #    and height of the preview window
-  # 2. Special preview window flag 'clear' is added to always completely
-  #    erase the preview window. This is similar to https://github.com/vifm/vifm/issues/588.
-  fzf --preview='
-    if file --mime-type {} | grep -qvF image/; then
-      bat --color=always {}
-    elif [[ -n $FZF_PREVIEW_WIDTH ]]; then
-      convert {} -resize ${FZF_PREVIEW_WIDTH}x${FZF_PREVIEW_HEIGHT} sixel:-
-    else
-      echo "Cannot display image data (unsupported platform)"
-    fi
-  ' --preview-window clear
-  ```
+- (Experimental) Sixel image support in preview window (not available on Windows)
+    - `$FZF_PREVIEW_PIXEL_WIDTH` and `$FZF_PREVIEW_PIXEL_HEIGHT` are set to
+      the pixel width and height of the preview window
+    - [bin/fzf-preview.sh](bin/fzf-preview.sh) is added to demonstrate how to
+      display an image using Kitty image protocol or Sixel. You can use it
+      like so:
+      ```sh
+      fzf --preview='fzf-preview.sh {}'
+      ```
 - Bug fixes
 
 0.43.0
 ------
 - (Experimental) Added support for Kitty image protocol in the preview window
+  (not available on Windows)
   ```sh
   fzf --preview='
     if file --mime-type {} | grep -qF image/; then