`ShadowRootInit.slotAssignment` should not work if ImperativeSlotAPIEnabled=false.
Pull request: https://github.com/WebKit/WebKit/pull/3098
Committed 253199@main (6587a3af61c6): <https://commits.webkit.org/253199@main> Reviewed commits have been landed. Closing PR #3098 and removing active labels.
<rdar://problem/98303092>