Support Windows Terminal progress UI #47

Windows Terminal added support for ConEmu's OSC 9;4 escape sequences a while ago to trigger various nice UI things in the terminal. In particular, one can show a progress bar in the task bar and on the tab for a given Julia instance. I think the best place to add support for that would be here? Essentially all one has to do is emit various escape sequences if on Windows, and it should all work.

Couple questions:

• My sense is that the current progress logging framework here fundamentally supports multiple progress bars a the same time? There is only one per terminal window, I'm a bit lost how we can handle that?
• One can also emit certain escape sequences to set an error and warning state for the terminal. I think it would be nice if for example the task bar button showed the red bar when there was an error, and also showed a non-progress-bar-but-something-is-running whenever the REPL is executing code that doesn't do any progress reporting. The problem is that stuff seems more like REPL integration, though... So I'm a bit lost where that kind of code should be...

Some screenshots follow.

Showing a 40% progress in the tab and on the taskbar:

Showing an error state on the taskbar:

Showing the "something is running" on the taskbar, note that this is animated and the little bar is constantly moving:

And then there is also a yellow state, not clear to me what we would use that for: