[WIP] Style engine: split methods to generate css and classnames

[ramonjd]

🚧

Maybe we don't actually need this.

The current generate functions are very block support-specific. Block supports require CSS and classnames.

Moreover, if we are to generate semantic classnames for block supports in the future, the array() return value is already there to accommodate.

It'd save us a loop too 😄

What?

Create two methods wp_style_engine_generate_classnames and wp_style_engine_generate_css, both of which return strings.

This avoids returning array( 'css' => '', 'classnames' => '' ), which doesn't apply in all contexts

Why?

How?

Testing Instructions

Screenshots or screencast