Use unicode placeholders to print images under kitty+tmux

[mfontanini]

This uses the unicode placeholder strategy under kitty to get images (including gifs) to work when running under tmux. Caveats:

• Gifs only seem to really work correctly under tmux 3.3a. 3.2a sometimes doesn't display them upon the first try.
• This doesn't currently detect kitty (or wezterm) correctly when running in tmux so if you're using tmux on either of those terminals that support the kitty protocol, you need to pass --image-protocol kitty-local (kitty-remote seems to not work great). I may add some better detection but at least this currently works.
• I tried to get this to work under the iterm protocol as well but I'm afraid it won't work. tmux has some buffer of 1MB that kicks in here and because the iterm protocol sends the entire image in a single b64 chunk, that can easily overflow for any non tiny image. If there's some way to send chunked images in iterm, this can be revisited.

Fixes #72