Rename variables for brevity

- Rename `$enable-prefers-reduced-motion-media-query` to `$enable-reduced-motion`
- Rename `$enable-pointer-cursor-for-buttons` to `$enable-button-pointers`

scss/_reboot.scss

@@ -463,7 +463,7 @@ button,
 [type="submit"] {
   -webkit-appearance: button; // 2
 
-  @if $enable-pointer-cursor-for-buttons {
+  @if $enable-button-pointers {
     &:not(:disabled) {
       cursor: pointer; // 3
     }