Block Themes: Add section styling via extended block style variations

Provide users with the ability to style entire sections of a page without
having to tediously reapply the same sets of styles.

This is done by extending block style variations to apply to nested blocks.

See WordPress/gutenberg#57908.

Fixes #61312.
Props aaronrobertshaw, talldanwp, ramonopoly, isabel_brison, andrewserong.

Built from https://develop.svn.wordpress.org/trunk@58264


git-svn-id: https://core.svn.wordpress.org/trunk@57727 1a063a9b-81f0-0310-95a4-ce76da25c4cd

Author: noisysocks