Remove T.log.withPromptPaused in favor of disabling the prompt automatically for every exclusive command

[lihaoyi]

We continue printing the ticker task-output header above the output for every task, so if you run more than one exclusive command like resolve _ + version you can see which block of output belongs to which command

[1/2] resolve
clean
init
inspect
path
plan
resolve
show
showNamed
shutdown
taskSpawningFutures
version
visualize
visualizePlan
[2/2] version
0.12.0-RC2-63-2851b8-DIRTYc324c032

As a result of re-using PrefixLogger and PrefixOutputStream, exclusive command output continues to be line-buffered, and cannot display sub-line terminal UIs such as loading bars. That could potentially be fixed in future

Tested manually via ./mill -i dist.launcher && (cd example/depth/tasks/7-forking-futures && ../../../../out/dist/launcher.dest/run -w resolve _ + version)