Put ShadowRootInit.slotAssignment behind a flag

https://bugs.webkit.org/show_bug.cgi?id=243654

Reviewed by Ryosuke Niwa.

`ShadowRootInit.slotAssignment` should not work if ImperativeSlotAPIEnabled=false.

* Source/WebCore/dom/ShadowRootInit.idl:

Canonical link: https://commits.webkit.org/253199@main

Source/WebCore/dom/ShadowRootInit.idl

@@ -26,5 +26,5 @@
 dictionary ShadowRootInit {
     required ShadowRootMode mode;
     boolean delegatesFocus = false;
-    SlotAssignmentMode slotAssignment = "named";
+    [EnabledBySetting=ImperativeSlotAPIEnabled] SlotAssignmentMode slotAssignment = "named";
 };