Add CSS syntax patch for: `<timeline-range-center-subject>`

### Link to the definition in the spec

https://drafts.csswg.org/pointer-animations-1/#typedef-timeline-range-center-subject

### Syntax of the CSS construct

```code
source | target
```

### Notes

Via #1647. The syntax patch should rather be defined with a `useValues` flag. Pending https://github.com/w3c/csswg-drafts/pull/13225 to fix the association of the values with the type in the spec.