Refactor material-next's Slider style implementation to use CSS Variables similar to Joy UI's Slider.

The goal of doing this is to make the style implementation DRYer. For example, adding a --md-comp-slider-track-size variable used for the track's height on horizontal sliders and the track's width on vertical sliders. This will also enable customization via CSS variables.

It should take inspiration from Joy UI's slider, using similar variables if applicable. It doesn't have to be a 1:1 copy; some variables in Joy UI might not be helpful in Material UI, and some might have to be added.

The variables should follow Material You's token syntax: --md-comp-slider-token-in-kebab-case.