fix(br): reinstate br-sm, br-lg CSS variables for fallback (#2037)

* fix(br): reinstate br-sm, br-lg CSS variables for fallback

* Add a todo

* add changeset

Author: dancormier

.changeset/salty-papers-sneeze.md

@@ -0,0 +1,5 @@
+---
+"@stackoverflow/stacks": patch
+---
+
+Add legacy sm, lg border radius custom properties

packages/stacks-classic/lib/exports/constants-helpers.less

@@ -31,9 +31,12 @@ body {
     --zi-modals:             9000;   //  Modals
 
     // Border Radius
-    --br-md:                 calc(var(--su-static4) + var(--su-static6)); // 10px;);
+    --br-md:                 calc(var(--su-static4) + var(--su-static6)); // 10px;
     --br-circle:             50%;
     --br-pill:               1000px;
+    // TODO SHINE Retaining legacy border radius for backward compatibility
+    --br-sm:                var(--br-md); // Retaining legacy border radius for backward compatibility
+    --br-lg:                var(--br-md); // Retaining legacy border radius for backward compatibility
 
     // Transition easings
     --te-smooth-slow:        cubic-bezier(0.25, 0.46, 0.45, 0.94);       // easeOutQuad