Discovered while triaging #17556
This PR improves the `color-mix(...)` polyfill to ensure it works when a
theme key links to another theme key via a `var(...)` call.
Imagine this setup:
```css
@theme {
--color-red: var(--color-red-500);
--color-red-500: oklch(63.7% 0.237 25.331);
}
@source inline("text-red/50");
````
Since `--color-red` will link to `--color-red-500` _which is also a
known theme variable_, we can inline the value of `--color-red-500` into
the fallback now:
```css
.text-red\\/50 {
color: var(--color-red);
}
@supports (color: color-mix(in lab, red, red)) {
.text-red\\/50 {
color: color-mix(in oklab, var(--color-red) 50%, transparent);
}
}
```
This will allow for slightly less confusing behavior.
The code added also handles recursive definitions where a color is
linking to another color that is again linking to the first one (by
adding a `Set` to keep track of already seen variable names).
## Test plan
- Added unit test
3e9cf87adf