Function formatter

cli/unstable_progress_bar_test.ts:132–139

(x)

    max: 10 * 1000,
    keepOpen: false,
    formatter(x) {
      called = true;
      if (lastTime != undefined) assertEquals(x.previousTime, lastTime);
      if (lastValue != undefined) assertEquals(x.previousValue, lastValue);
      lastTime = x.time;
      lastValue = x.value;
      return "";
    },
  });

  bar.value += 1000;