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

scss/helpers/_ratio.scss

@@ -6,7 +6,7 @@
 
   &::before {
     display: block;
-    padding-top: var(--#{$variable-prefix}aspect-ratio);
+    padding-top: var(--#{$prefix}aspect-ratio);
     content: "";
   }
 
@@ -21,6 +21,6 @@
 
 @each $key, $ratio in $aspect-ratios {
   .ratio-#{$key} {
-    --#{$variable-prefix}aspect-ratio: #{$ratio};
+    --#{$prefix}aspect-ratio: #{$ratio};
   }
 }