[Tiny PR] Multi selection: hide toolbar when multi selecting

[ellatrix]

Description

This was raised in #16835, though it has been the behaviour for a long time. It does make more sense to hide the block toolbar during multi block selection so that it doesn't interfere when selecting upwards.

How has this been tested?

Select two block dragging from the bottom to the top one. The toolbar should disappear when moving over the block boundary.

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR. .

[youknowriad]

I think there's a small update we can do that can improve the multi-selection feeling. While we're multi-selecting, if we can show the left border while we're selecting, I think it will clarify things more (that's the thing that indicates multi-selection for me right now :) )

Also, ideally, instead oh hiding the toolbar completely, we can show the right toolbar (the multi-selection one)

[ZebulanStanphill]

@youknowriad Yeah, it feels unresponsive to have the left border and multi-block toolbar not appear until shortly after you let go of the mouse button.

[ellatrix]

Closing this in favour of #18915.