Add Center Align Support to Embed Blocks

[thexavismith]

What?

Adding center align support to embed blocks

Why?

#15130

The issue was also identified here: Automattic/wp-calypso#38441

How?

The PR makes .wp-embed-block__wrapper a flex container and centers the content if the .aligncenter class is applied.

Testing Instructions

1. Create a new page or post
2. Add an embed block (i.e. Twitter)
3. Center align the block
4. Verify that the block is centered in the preview and frontend

Screenshots or screencast

[github-actions]

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[carolinan]

For twitter embeds, this works for me on the front but not in the block editor.
To test centering, we need to place the block in a container that is wider than regular content, for example a full width group block (Since with regular content width, center and "none" would look the same in the block editor).

Is there an easy way to test all embeds at once?

[carolinan]

Tagging this open issue: #15130

[thexavismith]

Centering the embed in the editor is a bit harder. On the frontend, it seems the embed is adding its actual width whereas on the editor side, the iframe is filling all the available space

[carolinan]

@smithtimmytim Hi, are you still working on this pull request?