Reduce the padding around the sibling inserter icon

[mapk]

This PR is so the icon doesn't interfere with other UI elements. Kind of fixes #16646. I wanted to try a delay on the Inserter, but animating the display attribute isn't possible with just CSS. I think that's why the Inserter uses the opacity attribute instead, but opacity still takes up space even when opacity is set to 0. This results in an element that still covers other UI elements.

Basically, I changed the padding around the + icon. It's not the most robust solution, so I'm open to alternatives.

How has this been tested?

Tested locally.

Screenshots

This PR

Master branch

Types of changes

Minor CSS change.

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.

[mapk]

A few more points I'd like to try in another PR as mentioned in #13571:

1. Move the sibling inserter below the selected block.
2. Only show sibling inserter when a block is selected. Similar to the proposed idea here: Move buttons are hard to use within a nested block #16617 (comment)

[kjellr]

Yep, this seems like a simple workaround and works in my testing. I think we can deploy this while we sort out those other, better solutions. 👍

[shaunandrews]

Thank you! The visual size of the inserted has always bugged me. This looks so much better.