[feature request] layout,syntax: Pie Chart

[wanglong001]

First of all, thank you for open-sourcing such a useful tool: d2 I also hope to have the function of drawing pie charts, I hope you can adopt it

as follows

myPie {
      shape: pie
      title: Key elements in Product X

      Calcium: 42.96
      Potassium : 50.05
      Magnesium : 10.01
      Iron :  5
}

Thanks again