[popover][positioner] Add JS access to `--available-width` and `--available-height`

## Summary

Expose `--available-width` and `--available-height` from Positioner to JavaScript via a prop or change handler. These values are currently available only as CSS variables, but accessing them in JS would unlock more flexible behavior. Maybe something like `onAvailableDimensionChange`?

## Motivation

I need these values in JavaScript to calculate a fixed height for an element rendering virtualized list inside. Having access to --available-height would allow me to dynamically adjust layout and improve performance. I tried to get values from CSS via refs and MutationObserver, but I got ugly code that didn't work completely. For some reason I get multiple cleanups from Positioner and fall into an infinite loop.