Layout: Try adding centralised attribute-based styles defined via classnames

[andrewserong]

What?

🚧 🚧 🚧 🚧 WIP: This is an exploration for now and is likely to change quite a bit 🚧 🚧 🚧 🚧

Following on from #40875 and as part of #39336, this PR looks at seeing if we can define those Layout attributes that are of a controlled set (e.g. content justification) in the theme.json layout definitions, and then output those definitions as preset-like rules.

Why?

Continue the de-duplication of style output generated by the Layout block support, and make it easier to add / define additional layout properties in the future.

How?

TBC

To-do

• Add all attributes to the centralised layout definitions
• Generate classnames in layout.php based on these definitions
• Generate CSS rules in the Theme JSON class based on these definitions

Testing Instructions

TBC

Screenshots or screencast

[andrewserong]

Pretty hacky, but I've had a go at the PHP implementation in:d68095a

Which results in the following output:

Next up:

• See if a similar sort of logic works for generating the classnames in layout.php
• See if a similar sort of logic works in the JS implementations (global styles, and classname generation in the editor)

[andrewserong]

Closing out this one now, as it's stale.