(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

bin/fzf-preview.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+#
+# The purpose of this script is to demonstrate how to preview a file or an
+# image in the preview window of fzf.
+
+file=$1
+type=$(file --mime-type "$file")
+
+if [[ ! $type =~ image/ ]]; then
+  # Sometimes bat is installed as batcat.
+  if command -v batcat > /dev/null; then
+    batname="batcat"
+  elif command -v bat > /dev/null; then
+    batname="bat"
+  else
+    cat "$1"
+    exit
+  fi
+
+  ${batname} --style="${BAT_STYLE:-numbers}" --color=always --pager=never -- "$file"
+elif [[ $KITTY_WINDOW_ID ]]; then
+  # 'memory' is the fastest option but if you want the image to be scrollable,
+  # you have to use 'stream'
+  kitty icat --clear --transfer-mode=memory --stdin=no --place="${FZF_PREVIEW_COLUMNS}x${FZF_PREVIEW_LINES}@0x0" "$file" | sed \$d
+  echo -en "\e[m"
+elif [[ -n $FZF_PREVIEW_PIXEL_WIDTH ]]; then
+  convert "$file" -resize "${FZF_PREVIEW_PIXEL_WIDTH}x${FZF_PREVIEW_PIXEL_HEIGHT}>" -dither FloydSteinberg sixel:-
+else
+  file "$file"
+fi