Block: side inserter in Popover

[ellatrix]

Description

This PR moves the "side inserter" to the block controls popover.
It also consistently places it on the right side. Currently we place it on the right side on mobile and in nested context, but on the left on wide screens if the block is at the root. See #19045 (comment).

Either we should consistently place it on the right side, or we could consider moving it to the left side but within the block boundary. I picked the first option because we're already doing that on mobile and for nested paragraphs. If needed, we can explore the inside-left option separately.

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. .

[jasmussen]

Great work reducing the markup!

You raise a good question: what do we do about the plus button in an empty paragraph?

Here's how it looks on desktop when not nested:

Here's the same, just inside a group:

As you note, the latter is also the case on mobile.

Recently, the "recent blocks" shortcuts were removed, which make the remaining discrepancy all the more visible. In effort to keep things moving, and because we can always revisit, I would suggest it's okay to show the plus on the right, everywhere. And if something comes up, or if a better idea emerges from #18667, we can revisit!

[jasmussen]

Related: #10519

[ellatrix]

Let's merge and explore more ideas. Another solution could be to show the inserter next to the caret on the right.