[css-anchor-position] Make `position-area` discretely animatable, for now.

[bramus]

The css-anchor-position-1 spec defines the animation type of position-area as:

TBD

This is leading to inconsistencies across browsers:

• Safari and Chrome: the property animates discretely
• Firefox: the property does not animate

Still allowing a possible future custom description for how it should animate (the magic in #9598 (comment) for the L2 spec), I think we should define it as discretely animatable until then.

This would remove the cross-browser inconsistency and also unlock animating from one position-area to another using View Transitions with a StyleObserver, as shown in this demo (which currently doesn’t work in Firefox): https://codepen.io/bramus/full/myrJYQz/b6aace5cc714e3147f81a2a1b4ed10f4

[bramus]

I checked Stylo and position-area is defined as discretely animatable (so the behavior I saw is most likely a bug)

We should still update the spec though, so that it lines up with what browsers have implemented.