Block: reposition tabbable inserter

[ellatrix]

Description

Unblocks #19322.

This PR moves the tabbable inserter before the block toolbar.
In the future we can polish this by perhaps centring the button and add one below the block as well.

Why before the toolbar? Because technically you don't leave the block until you exit the block toolbar, and the inserter buttons make most sense on the edges of the block, before and after the block.

The following button normally invisible. It's only visible on focus.

How has this been tested?

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]

LGTM 👍

[ellatrix]

Chatted with @MarcoZehe and we agreed that it makes sense for the inserter to be positioned before the block toolbar (in tab order) as this makes it more clearer that it's not a block control. if this is not good, we can always change the tab position but keep the visual position.