dillon1000/react

Always skip unmounted/unmounting error boundaries (#19627)

The behavior of error boundaries for passive effects that throw during cleanup was recently changed so that React ignores boundaries which are also unmounting in favor of still-mounted boundaries. This commit implements that same behavior for layout effects (useLayoutEffect, componentWillUnmount, and ref-detachment).

The new, skip-unmounting-boundaries behavior is behind a feature flag (`skipUnmountedBoundaries`).