Add SlotAssignmentMode to ShadowRootInit and ShadowRoot
Pull request: https://github.com/WebKit/WebKit/pull/3081
Committed 253198@main (d94de81283e2): <https://commits.webkit.org/253198@main> Reviewed commits have been landed. Closing PR #3081 and removing active labels.
<rdar://problem/98294441>