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.

この issue にはファイル、テスト、エントリーポイントが記載されていません。まず Graphite における図形と corner radius の実装箇所を特定し、次に corner smoothing をどのように表現し、公開すべきかを判断してください。完了の条件は、ユーザーが要求された squircle 効果を持つ図形を作成でき、その動作が適切なテストでカバーされていることです。

索引モデルが issue の本文から書いたものです。