[accordion][collapsible] Expose panel sizes as unitless values

# Feature request

## Summary

Base UI currently exposes `--accordion-panel-height` and `--accordion-panel-width` as CSS variables, which are useful for transitioning the panel's size. However, these values include `px`, which makes them less flexible for cases like calculating dynamic transition-duration.

It would be helpful if Base UI also exposed unitless size values (e.g. `--accordion-panel-height-value`) so that consumers could use them in `calc()`.

## Motivation

I would like to create "duration by distance" transitions for accordion panels, where the animation duration scales with panel size.