Convert theme to latest "block" way

Add "block"-compatibility to the theme: theme.json, expose various settings for default WP Gutenberg blocks, synchronize them between Elementor and WP's (like global fonts, colors, paddings/margins/gaps), etc.

This way we can create complicated custom things in Elementor and allow content managers to easily edit things on front-end while our designs are in shape and synced.

Ref: https://make.wordpress.org/core/2022/01/04/block-themes-a-new-way-to-build-themes-in-wordpress-5-9/