feat(api): support node with callback children

[pearmini]

chart.options({
  type: 'repeatMatrix',
  width: 480,
  height: 480,
  paddingLeft: 50,
  paddingBottom: 50,
  data: {
    type: 'fetch',
    value: 'data/penguins.csv',
  },
  encode: {
    position: ['culmen_length_mm', 'culmen_depth_mm'],
    color: 'species', // 需要显式指定颜色编码，否则无法推断
  },
  children: () => ({ // 回调
    type: 'point',
    encode: { color: 'species' },
  }),
});

[coveralls]

Pull Request Test Coverage Report for Build 6297209889

• 17 of 18 (94.44%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.009%) to 88.193%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/api/utils.ts	17	18	94.44%

Totals
Change from base Build 6296221511:	0.009%
Covered Lines:	10029
Relevant Lines:	11013

---

💛 - Coveralls