Implement document.scrollingElement (#35994)

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This implements `document.scrollingElement`
(https://drafts.csswg.org/cssom-view/#dom-document-scrollingelement).

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #35700
- [x] There are tests for these changes

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---------

Signed-off-by: JimmyDdotEXE <50691404+JimmyDdotEXE@users.noreply.github.com>

components/script_bindings/webidls/Document.webidl

@@ -204,6 +204,16 @@ partial interface Document {
 
 Document includes DocumentOrShadowRoot;
 
+// https://drafts.csswg.org/cssom-view/#extensions-to-the-document-interface
+partial interface Document {
+  // CaretPosition? caretPositionFromPoint(double x, double y, optional CaretPositionFromPointOptions options = {});
+  readonly attribute Element? scrollingElement;
+};
+
+// dictionary CaretPositionFromPointOptions {
+//   sequence<ShadowRoot> shadowRoots = [];
+// };
+
 // https://w3c.github.io/selection-api/#dom-document
 partial interface Document {
   Selection? getSelection();