Built-in support for OSC9;4

[zkat]

Various terminals now support OSC9;4, which lets you update terminal status bars themselves with progress details. It would be wonderful if something like this were built-in to indicatif, and probably isn't a lot of work to integrate. See microsoft/terminal#6700

[djc]

I'm open to having support for this, but unlikely to be motivated to implement it myself.

[chris-laplante]

Yes, please feel free to submit a PR!

[djc]

FWIW, if someone wants to fund work on this, happy to have a conversation.

[PRO-2684]

Implementing OSC 9;4 is trivia, when there's no more than one progress bar at the same time. When there's multiple, we should either:

• Devise a way to determine which one of them controls the outputting of OSC 9;4 sequences;
• Or provide a way for the caller to determine the total progress.

Regarding this, we couldn't simply extend ProgressBar, making it outputting OSC 9;4 sequences when refreshed. Of course, we could simply warn and trust the end user to make sure only one of the progress bars is producing OSC 9;4 sequences, but a better solution should utilize compile-time checking. I'd be happy to implement this when I'm free if such a solution has been found.