corner-shape support

## What problem does this solve or what need does it fill?

https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/corner-shape

## What solution would you like?

API something like:

```rust
pub enum CornerShape {
Round,
Bevel,
Notch,
Scoop,
Squircle,
Superellipse(f32),
}
```

Initial PR could just support `Round` and `Bevel` or something.

`CornerShape` should also be supported by picking, so interactions outside of the inset border corner-shapes are ignored. I'd be happy for the inital PR to be rendering-only and update picking in a follow up though.

Needs support for elliptical border radius first: #24782, which already has an implementation PR: https://github.com/bevyengine/bevy/pull/24779 which needs reviews.

Start by reviewing issue #24782 and its implementation PR #24779, since elliptical border-radius support is a prerequisite. Then trace Bevy's rendering and picking paths for inset border corners. Done means supporting the agreed initial CornerShape variants in rendering, with picking included if the scope permits.

Written by the indexing model from the issue text.