[popover] Consider adding the ability to set some `Positioner` props via CSS variables

# Feature request

## Summary

The following props of `Popover.Positioner` (and other `Positioner` components) would make sense to change at different breakpoints, which would be easy to do with media queries if they could be set via CSS variables / custom properties (e.g. `--side-offset: 16px`):


## Examples in other libraries

Give me a couple months and I'll have an example in my own library :D

## Motivation

Of course it's possible to change the prop dynamically in JS, but CSS is a much more natural place to do this sort of conditional styles. Consider this example with container queries in Tailwind CSS:

```jsx

Thank you!