Add UI to Global styles to allow managing spacing presets

[matiasbenedetto]

What?

Add UI to Global styles to allow modification and creation of spacing scale preset values.

Why?

This will allow users to edit the spacing presets using the editor.

[christopherabalos]

My apologies if this has been covered somewhere else but I wanted to mention it on this issue due to cross concerns.

I make use of clamp in spacings. It's been working great to dynamically adjust content density. If this could be accounted for and handled exactly how font sizes work, it would be amazing. This would require an adjustment to theme.json schema to accept a min and max value. The UI would then be very similar to the new fonts UI.