Close #2598 (#2630)

* Close #2598

- Add $card-radius variable
- Default to 0 (no braking change)
- Update variables/card.json

* Fix per @jgthms

- Could be a small breaking change
   since `overflow: hidden` is added to card

docs/_data/variables/components/card.json

@@ -19,6 +19,11 @@
       "value": "0 0.5em 1em -0.125em rgba($scheme-invert, 0.1), 0 0px 0 1px rgba($scheme-invert, 0.02)",
       "type": "size"
     },
+    "$card-radius": {
+      "name": "$card-radius",
+      "value": "0",
+      "type": "size"
+    },
     "$card-header-background-color": {
       "name": "$card-header-background-color",
       "value": "transparent",
@@ -98,4 +103,4 @@
     "$card-media-margin"
   ],
   "file_path": "components/card.sass"
-}
+}