Add additional root variables, rename `$variable-prefix` to `$prefix` (#35981)

* Add additional root variables, rename $variable-prefix to $prefix

- Adds new root CSS variables for border-radius, border-width, border-color, and border-style
- Adds new root CSS variables for heading-color, link-colors, code color, and highlight color
- Replaces most instances of Sass variables (for border-radius, border-color, border-style, and border-width) for CSS variables inside _variables.scss
- Updates $mark-padding to be an even pixel number
- Renames $variable-prefix to $prefix throughout

* Bundlewatch