Add support for Section Styles

[mikachan]

Section Styles have been introduced in Gutenberg 18.5 and will be included with WP 6.6, and they'll also be a part of the next default theme, Twenty Twenty-Five.

We should consider ways these can be managed via the UI, such as:

• Allowing section styles to be created and edited in the Editor
• Allowing section styles to be exported
• Allowing block style variations to be exported

[richtabor]

Perhaps it can be abstracted a bit to allow for exporting block style variations (assuming we can eventually create block style variations in Gutenberg).

[t-hamano]

This issue is probably related to #698 too.