[Feature] crosshiar should follow tooltip

[xile611]

What problem does this feature solve?

const spec = {
  type: 'line',
  data: {
    values: [
      {
        time: '2:00',
        value: 38
      },
      {
        time: '4:00',
        value: 56
      },
     
      {
        time: '10:00',
        value: 36
      },
      {
        time: '14:00',
        value: 24
      },
      {
        time: '16:00',
        value: 44
      },
      {
        time: '22:00',
        value: 22
      }
    ]
  },
  xField: 'time',
  yField: 'value',
  line: {
    style: {
      curveType: 'monotone'
    }
  },
  axes: [{
    orient: 'bottom',
    type: 'band',
    domain: new Array(24).fill(0).map((entry, index) => {
      return `${index}:00`
    })
  }]
};

const vchart = new VChart(spec, { dom: CONTAINER_ID });
vchart.renderSync();

// Just for the convenience of console debugging, DO NOT COPY!
window['vchart'] = vchart;

1. tooltip现在只在有数据的点展示，但是crosshair 在没数据的点也会展示，希望crosshair 和tooltip同步

What does the proposed API look like?

{
  followTooltip: true
}