Fix: Some default block appender styles are set on the editpost module

[jorgefilipecosta]

This PR moves styles from the edit-post module into the block editor module. These styles are required for the default block appender work as expected, and that component is part of the block editor module so these styles should be set there.

This problem is affecting the block widgets screen.

Related: #16601

How has this been tested?

I enabled the experimental widgets screen in the settings.
I went to the experimental widgets screen.
Deleted all the content in one of the block areas.
I added an image block, and I verified the insert button of the default block appender is visible, in master it is not.

[mapk]

@jorgefilipecosta thanks for your work on this! I wanted to reply to your comment from the issue on this PR.

It solves the issue by providing the same appender as the normal post editor.
If for this scenario having a custom appender is a must, I guess we can follow up by allowing the block editor module to be more customizable, similar to what we allow now for inner blocks.

I think in context of the editor, having the smaller appender is good because the editor is meant for writing a flow of content. I believe users think of Widget Areas differently. These feel more block-related where the user adds blocks of particular content to show in a sidebar or whatever.

That being said, I'd like to go with a larger custom appender. The same we use in the Columns block would be fantastic.

[youknowriad]

Does this style belong here or in the paragraph block instead?

[jorgefilipecosta]

Yes, I think so, I moved the styles to the paragraph block 👍

[jorgefilipecosta]

Hi @mapk the appender we use for columns and groups was added in #16971.
This PR is still relevant because it solves a problem happening on Block Editor using the default appender logic.