Accordion item content: add width and height css variables

[bogdan0083]

🚀 Feature request

It would be cool to be able to get item content width and height through css variables. This would allow us to create slide animations similar to new Collapsible component.

🧱 Problem Statement / Justification

Right now it's quite a challenge to create slide animations on item content. We need to use Transition component and compute size by hand. Having css variables on each item content would make life easier.

✅ Proposed solution or API

The solution would be to use the approach similar to Collapsible component. Compute size of each content on the initial mount and recompute it on open/close.

[segunadebayo]

The accordion and collapsible can be composed together in the userland code as shown here:
41d8ba7

We'll update the docs to include patterns like this.