LibWeb: Allow custom properties in CSSStyleDeclaration.removeProperty()

Author: https://github.com/kalenikaliaksandr
Commit: ac5699c8fc
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2460
Reviewed-by: https://github.com/AtkinsSJ ✅

Tests/LibWeb/Text/input/css/CSSStyleDeclaration-custom-properties.html

@@ -16,5 +16,8 @@
         document.body.appendChild(div);
 
         println(getComputedStyle(nested).backgroundColor);
+
+        div.style.removeProperty("--redcolor");
+        println(getComputedStyle(nested).backgroundColor);
     });
 </script>