Style engine: return declarations in array response

[ramonjd]

What?

This PR adds uncompiled CSS declarations to the return values of get_block_supports_styles so that consumers can use the array values (if they want)

Props to @andrewserong for the idea over at #42085 (review)

Why?

Consumers might want to access parsed declarations but not necessarily the compiled version.

Parsed declarations will have already converted var:preset|slug values to var(--wp--preset--slug).

Real world use case: #42085 (review)

How?

As well as the compiled CSS, wp_style_engine_get_block_supports_styles() will return the array of css declarations as stored in WP_Style_Engine_CSS_Declarations.

wp_style_engine_get_block_supports_styles( 
    array(
        'border' => array(
            'bottom' => array(
                'color'  => 'var:preset|color|terrible-lizard',
            ),
        ),
    )
);

/* Returns:
array(
    'css'          => 'border-bottom-color: var(--wp--preset--color--terrible-lizard);',
    'declarations' => array(
        'border-bottom-color' => 'var(--wp--preset--color--terrible-lizard)',
    ),
)
*/

Testing Instructions

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

[andrewserong]

Looks good to me, tests are passing, and smoke tested quickly that it doesn't break anything.

Thanks for putting this together so quickly!