[Feature] optimize the pie label layout for few label

What problem does this feature solve?

• 目前在饼图标签数据较少同时标签内容较大（多行）的情况下，饼图标签的布局效果不佳，需要优化；
• 并且在顶部以及底部的标签容易由于空间不足而无法显示。

example:

const spec = {
  type: 'pie',
  data: [
    {
      id: 'id0',
      values: [
        { type: 'oxygen', value: '4600.60' },
        { type: 'silicon', value: '27.72' },
        { type: 'aluminum', value: '8.13' },
        { type: 'iron', value: '5' },
        { type: 'calcium', value: '3.63' },
        { type: 'sodium', value: '2.83' },
        { type: 'potassium', value: '2.59' },
        { type: 'others', value: '3.5' }
      ]
    }
  ],
  outerRadius: 0.9,
  valueField: 'value',
  categoryField: 'type',
  label: {
    visible: true,
    style: {
      text: ['aaa', 'aaa', 'aaa', 'aaa', 'aaa']
    }
  },
  tooltip: {
    mark: {
      content: [
        {
          key: datum => datum['type'],
          value: datum => datum['value'] + '%'
        }
      ]
    }
  }
};

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

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

What does the proposed API look like?

null

Reproduce the supplied pie-chart example and trace the pie label layout entry point in VChart. Done means labels with few, multiline values remain visible and are laid out correctly near the top and bottom of the chart, with appropriate regression coverage.