[Feature] option for levels radius % to depend on sunburst radius instead of viewport

[helgasoft]

What problem does this feature solve?

Sunburst levels.radius could be defined in pixels or percentages of viewport size. Both options make levels' size static - Demo.

A new convenient and responsive option would be to allow percentages to depend on current sunburst radius. This will allow a command like myChart.setOption({ series: [{ radius: [0, '80%'] }] }); to correctly scale the entire chart. Also helps with zooming sunburst charts since roam property is now missing. See #19370

What does the proposed API look like?

levels: [ 
    { radius: '20%'  },  // percentage of sunburst radius
    { },  { },  // these 2 levels will adapt to fit in remaining space (2 * 5% = 10%)
    { radius: '70%'  },  // percentage of sunburst radius
]

At present, radius is defined as a number or an array, so adding a string type wont break existing code. This also avoids adding a new option property.