State-preserving atomic move integration

This complements whatwg/dom#1307. It integrates "moving steps", which allow HTML elements to react to atomic node moves without the use of the "insertion steps" or "removing steps". Those intentionally fail to preserve most state and are therefore not always suitable for a node "move". The changes in this PR are inspired by the audit carried out in https://docs.google.com/document/d/1qfYyvdK4zhzloABKeh0K1lHPm-SpnEcsWEE9UdDuoMk/edit.

Author: domfarolino