stacked option for radar chart

### Feature Proposal

A stacked option in radar chart for easier usage like the line/bar chart.

The line and bar chart has an option; `options.scales.x.stacked: true`. A radar chart can stack, but not as easy as setting a boolean. https://www.chartjs.org/docs/latest/samples/area/radar.html

It would be much more convenient to use `options.scales.r.stacked: true` for the radar chart, just like the line / bar chart

### Possible Implementation

Add the option `stacked` to the scale of radar chart.

`options.scales.r.stacked: boolean`