Refactor style properties computation in WP Theme JSON class

[spacedmonkey]

The method compute_style_properties() in class-wp-theme-json.php has been refactored for more efficient theme style computation. The flow has been improved by reordering conditions and breaking down complex ones into simpler bits. Now the code is easier to read, and the handling of protected properties and root styles are clearer.

Backport from #6707

Trac ticket: https://core.trac.wordpress.org/ticket/59595

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