Feature request: Add Sankey Chart rounded corners

Would love to see this feature to work with sankey charts. Similar to this version

https://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/highcharts/plotoptions/sankey-outgoing

But not arrowed shaped, rather rounded ends.

Start by reviewing the repository's JavaScript implementation and the linked Highcharts Sankey example to locate how Sankey node and link shapes are rendered. Done means Sankey charts support rounded, non-arrow-shaped ends as illustrated by the request, with the relevant behavior verified in the project's available checks.

Written by the indexing model from the issue text.