Full Site Editing: Add experimental SiteDescription block

[Copons]

Description

Add a SiteDescription block similar to SiteTitle but with styling attributes like text and background colors, text and block alignments, and font size.

Note: this depends on #18238 which enhances the PlainText component with a few RichText-like features. Should we decide the PlainText refactor is not worth it, it's fairly easy to change this to use RichText without allowed formats (see the WordPress.com implementation).

How has this been tested?

Tested on the Docker environment with the Full Site Editing experiment enabled.
Since the FSE experiment replaces the theme, to test this on the front end is necessary to create a template (Appearance > Templates) with index as slug, that will render as the theme index.php file.

Screenshots

Editor	Front End

Note: the font size difference is caused by the theme (Twenty Twenty). For some reasons, the larger font size is 32px in the editor and 42px in the front end.

Types of changes

New feature (non-breaking change which adds functionality)

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.

[Addison-Stavlo]

I have tested these changes on my docker environment.

This block seems to work well in both the editor and front end running the Twenty Twenty theme.

I am a little unfamiliar on how theme support for FSE works here, but I do notice if I switch to an older theme like 'Twenty Eleven' that this block is still accessible but does not allow for much customization such as width, etc. Is it worth restricting access to blocks like this to FSE enabled themes only? (or are all themes technically FSE enabled when we run the experimental plugin and some just don't have as much support as others?)

[Copons]

@Addison-Stavlo Thanks for reviewing! 🙇

Currently Core FSE don't really have a theme/front end side yet, and Core FSE blocks are only gated behind the (Core) FSE experiment.

(Also please notice that this PR depends on #18361)

[carolinan]

@Copons is this still blocked?

[Copons]

@carolinan Things have changed a lot in the last many months, and I'm afraid I might mess up something while rebasing.
I'll close this and reopen a new one!

[Copons]

Replaced by #23788