Style engine: rename 'css_var' option property to 'convert_vars_to_classnames'

[ramonjd]

What?

This PR renames css_var to convert_vars_to_classnames.

Why?

The style engine returns classnames in its output.

In the case of block supports, classnames are generated from var:preset|{$css_property}|{$slug} values and, in the resulting CSS, we skip generating var(--wp--preset--{$css_property}--{$slug}).

At the moment we have a css_var option to tell the style engine to compile CSS var values in CSS declarations.

This should, however, be the default.

Block supports are the only places where we need to skip CSS vars and return corresponding classnames.

So instead, block supports can use the flag convert_vars_to_classnames instead of everywhere else using css_var, which had a confusing name anyway.

How?

Renaming css_var to convert_vars_to_classnames

Testing Instructions

Run the tests!

npm run test-unit-php
npm run test-unit-php /var/www/html/wp-content/plugins/gutenberg/packages/style-engine/phpunit/class-wp-style-engine-test.php

[glendaviesnz]

Ah, that makes more sense - thanks for this tidy up - tests well for me, happy to 🚢 once that failing test is sorted.