LibWeb: Add DOMRect and Element.getBoundingClientRect()

This marks our entry into the Web::Geometry namespace, based on the
"Geometry" spec at https://drafts.fxtf.org/geometry/

Userland/Libraries/LibWeb/DOM/Element.idl

@@ -34,4 +34,7 @@ interface Element : Node {
     ArrayFromVector querySelectorAll(DOMString selectors);
 
     [SameObject] readonly attribute HTMLCollection children;
+
+    DOMRect getBoundingClientRect();
+
 };