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.

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