Squircle / Corner Smoothing Support

I understand that this may be a very low priority, and it is a very subtle effect but in modern design it can often help massively.

Editors such as Figma have support for Corner Smoothing (sometimes the shape is referred to as a squircle) which blends the border radius to look more organic and smooth. To me this is one of those subtle features that I use a large amount, and it would be amazing to have this in an open-source project like this :) Thanks for the amazing work so far, I've just picked this editor up and it has potential


Image sourced from [a random site](https://cedarstudioswebdesign.com/blog/rounded-corners-vs-square/) from google as a visual example.

The issue names no files, tests, or entry points. Begin by locating the shape and corner-radius implementation in Graphite, then determine how corner smoothing should be represented and exposed. Done means users can create shapes with the requested squircle effect and the behavior is covered by appropriate tests.

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