Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

Reduce the changes of misjudging the type, e.g., when file is under an `image/`
directory.

Close #3822

  Output encoding:
    -f, --format=FORMAT  Set output format; one of [iterm, kitty, sixels,
                       symbols]. Iterm, kitty and sixels yield much higher
                       quality but enjoy limited support. Symbols mode yields
                       beautiful character art.

* Use `--unicode-placeholder` for consistent result in and out of tmux
* Use updated version of junegunn/go-runewidth that handles diacritics
  used in Kitty Unicode placeholder

Close #3567

Close #1102

  fzf --preview 'imgcat -W $FZF_PREVIEW_COLUMNS -H $FZF_PREVIEW_LINES {}'

Notes:
* There is no good way to determine the height of the rendered image,
  so we assume that the image takes the full height of the preview
  window. So the image cannot be displayed with the other text.
* fzf-preview.sh script was updated to use `imgcat` if it's available
  but `chafa` is not.
* iTerm2 also supports Sixel, so adding support for this protocol is not
  quite necessary but it renders animated GIFs much better (e.g. looping).

So that it can determine if it should subtract 1 from $FZF_PREVIEW_LINES
to avoid scrolling issue of Sixel image that touches the bottom of the
screen.