Improve Mill command line status reporting UI to display concurrent tasks

[lihaoyi]

Now that parallelism is enabled by default, the previous "ticker" mechanism for reporting progress is no longer sufficient to tell the user what the build is doing.

SBT, Gradle, and Bazel all have nice terminal UIs with a multi-line status bar showing all tasks in progress at any point in time, although Maven doesn't have one. Mill should have one as well

Gradle

...logs...
<===========--> 86% EXECUTING [36s]
> :kotlinTest:test > 0 tests completed
> :inlineTest:test > 54 tests completed
> :kotlinReleaseCoroutinesTest:test > Executing test org.mockito.kotlin.SuspendTest
> :osgi-test:test > 0 tests completed
> :checkstyleMain
> :programmatic-test:test > 1 test completed
> :module-test:test > 21 tests completed, 17 skipped
> :kotlinReleaseCoroutinesTest:test > 0 tests completed
> :memory-test:test > 1 test completed
> :compileTestJava
> :memory-test:test > Executing test org...ShouldNotStarveMemoryOnLargeStackTraceInvocationsTest
> :subclass:test > 0 tests completed
> :inlineTest:test > Executing test org.mockitoinline.StressTest
> :osgi-test:test > Executing test org.mockito.osgitest.OsgiTest
> :module-test:test > Executing test org.mockito.moduletest.ModuleHandlingTest
> :programmatic-test:test > Executing test org.mockito.ProgrammaticMockMakerTest
> :kotlinTest:test > Executing test org.mockito.kotlin.FunctionTest
> IDLE
> :subclass:test > Executing test org.mockitosubclass.PluginTest

SBT

...logs...

  | => gatling-http-java / update 0s
  | => gatling-http-client / Compile / compileIncremental 0s
  | => gatling-jdbc-java / update 0s
  | => gatling-redis-java / update 1s
  | => gatling-test-framework / scalaCompilerBridgeBinaryJar 1s
  | => docSamples / scalaCompilerBridgeBinaryJar 3s
  | => gatling-commons / Compile / compileIncremental 3s
  | => gatling-jsonpath / Compile / compileIncremental 3s
  | => gatling-quicklens / Compile / compileIncremental 3s