LibWeb: Allow custom properties in CSSStyleDeclaration.getPropertyValue

Author: https://github.com/kalenikaliaksandr
Commit: ce26e5d757
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2460
Reviewed-by: https://github.com/AtkinsSJ ✅

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

@@ -5,6 +5,7 @@
     test(() => {
         const div = document.createElement('div');
         div.style.setProperty("--redcolor", "red");
+        println(`style.getPropertyValue(--redcolor)=${div.style.getPropertyValue("--redcolor")}`);
 
         const nested = document.createElement('div');
         nested.style["backgroundColor"] = "var(--redcolor)";