LibWeb: Implement Document.rootElement (SVG2 extension to Document)

Author: https://github.com/awesomekling
Commit: 879cba762b
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4775

Libraries/LibWeb/DOM/Document.idl

@@ -156,6 +156,9 @@ interface Document : Node {
     // special event handler IDL attributes that only apply to Document objects
     [LegacyLenientThis] attribute EventHandler onreadystatechange;
     attribute EventHandler onvisibilitychange;
+
+    // https://www.w3.org/TR/SVG2/struct.html#InterfaceDocumentExtensions
+    readonly attribute SVGSVGElement? rootElement;
 };
 
 dictionary ElementCreationOptions {