- 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