Style engine: add handling of spacing presets

[glendaviesnz]

What?

Adds handling of conversion of spacing preset values to css vars to style engine

Why?

#41527 adds the spacing presets to theme.json, as the first part of implementing standardised design tokens for spacing as detailed in #39371

How?

Extends the existing preset var handling added for the processing of color presets.

Testing Instructions

• Add the following block markup in the edit code view and save and view in the frontend:

<!-- wp:site-title {"style":{"spacing":{"padding":{"top":"var:preset|spacing|70","right":"var:preset|spacing|50","bottom":"var:preset|spacing|70","left":"var:preset|spacing|50"}},"elements":{"link":{"color":{"text":"var:preset|color|background"}}}},"backgroundColor":"vivid-red"} /-->

• Check in the frontend that the block markup has converted the var:preset|space|20 values into var(--wp--preset--spacing--20); in the block styles

N.B. The frontend styles markup in this PR is not an indication of the final marking for spacing presets. The final implementation may use utility classes as noted in #39371, but the fact that we are generating these styles dynamically on the frontend means we can modify the output relatively easily as we go.

Screenshots or screencast

[glendaviesnz]

@ramonjd @andrewserong - have moved the shared css var code into a shared method. I will add some tests before I take it out of draft.

[andrewserong]

Thanks Glen, that's looking cleaner now with the additional function 👍

Just left a question about naming of the placeholder for the CSS var, but this is looking close to me!

[glendaviesnz]

Replaced by #41990