Multi-Selection: Arrow Up/Down when all blocks multi-selected

[aduth]

Regression of #3745

Steps to Reproduce:

1. Create a new post
2. Insert a few blocks
3. Multi-select all of them
4. Press Arrow Up or Arrow Down

Expected:

Focus transitions to the title (up) or default block appender (down).

Actual:

Nothing happens.

Implementation Suggestion:

This logic in WritingFlow needs to be made less specific to blocks and instead reuse getClosestTabbable (enhanced in #5513):

gutenberg/editor/components/writing-flow/index.js

Lines 115 to 122 in 6332d40

	moveSelection( isReverse ) {
	const { previousBlockUid, nextBlockUid } = this.props;
	const focusedBlockUid = isReverse ? previousBlockUid : nextBlockUid;
	if ( focusedBlockUid ) {
	this.props.onSelectBlock( focusedBlockUid );
	}
	}

1. Get block focus tab stop node for extent in direction (up? first in multi-selection, conscious of directionality of selection)
2. this.getClosestTabbable( target, isReverse )
3. placeCaretAtHorizontalEdge( closestTabbable, isReverse );

[aduth]

Tested again. Now something does happen. It moves the caret to the first or last block for up or down respectively, which seems like a good behavior mimicking that of a horizontal selection collapsing.