Add a native CSS outline to gpui

[yu7400ki]

gpui has border but no CSS-style outline. I'd like to propose adding one — mainly for layout-free focus rings.

A focus ring should appear on :focus-visible without shifting content. border can't do that: taffy reserves the border box, so toggling a focus border nudges the layout. There's no primitive that draws a ring outside the box without affecting layout.

A native outline — uniform width/color/offset/style, drawn outside the border box, no layout effect, and composable with hover/focus_visible like border — would fill that gap and map onto Tailwind's outline-*:

div().focus_visible(|s| s.outline_2().outline_color(rgb(0x3d5a80)).outline_offset_2())

It looks cheap to add (reuses the existing rounded-rect border quad; no shader or taffy changes). Happy to open a PR if this sounds reasonable.

[yu7400ki]

Opened a PR for this: #59425