Add support for Sixels in integrated terminal

[marvinruder]

This PR adds support for Sixels to the integrated terminal, as proposed in #171666.

[marvinruder]

@microsoft-github-policy-service agree

[marvinruder]

I am confused regarding the change of yarn’s lockfile – “Prevent yarn.lock changes in PRs” disallows it, but “Monaco Editor checks” cannot pass without an updated lockfile since a dependency was added in the package.json and the job runs yarn with --frozen-lockfile.

[Tyriar]

I am confused regarding the change of yarn’s lockfile

We don't allow anyone external to make changes to the lockfile as it's difficult to review and someone could use that as a way to inject malicious packages under the rader.

I've been holding off adoption of this because of:

Sixel support and image handling in xterm.js is considered beta quality.

I believe it uses a bunch of internal APIs currently which are likely to break across version updates which would impact our agility negatively.

The discussion around all image support is tracked in xtermjs/xterm.js#614, I think the main blocker is figuring out how to have non-text buffer lines in xterm.js