feat: make spacing property as optional

[mlecoq]

Description

The goal is to be able to have a theme without spacing (like done for borderRadii and zIndices) to be able to hard code them everywhere

Reviewers’ hat-rack 🎩

• spacing props

Checklist

• I have added a changelog entry following the Keep a Changelog guidelines.

[aleclarson]

I published my fork, @alloc/restyle, which does this but for all theme values. So colors, spacing, …etc can be arbitrary values.