Add support for Sixels in integrated terminal

[marvinruder]

I would like the integrated terminal of VS Code to support Sixels for displaying graphics.

Since the integrated terminal is based on xterm.js, for which a very simple add-on xterm-addon-image exists, this feature can be integrated without much effort.

(The topic of images in the integrated terminal appears to have been discussed briefly in #65283, but the requested feature there seems to have been related to a different and proprietary image format of iTerm 2. Sixels on the other hand are much more common, having been in existence since the era of hardware terminals and being supported by terminal emulators such as xterm.)

[Tyriar]

Closing in favor of xtermjs/xterm.js#614, see #171667 (review)