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, and border-width) for CSS variables inside _variables.scss
• Updates $mark-padding to be an even pixel number
• Renames $variable-prefix to $prefix throughout

Cherry-picked from https://github.com/twbs/bootstrap/pull/35736. /cc @XhmikosR