Convert border utilities to CSS variables

• Updates the utilities mixin to check for specific CSS variable names via css-variable
• Bonus fix: we now prevent local variables for 0 value utilities (e.g., .border-top-0 no longer sets --bs-border-opacity: 1
• Adds new .border-opacity-* classes
• Adds new root variables: --bs-border-color, --bs-border-style, --bs-border-width
• Documents the new variable changes
• Replaces instances of the Sass variables with their new CSS variables

Todo:

• Clean up language and comments in the diff