Gutenberg: Convert to child theme

[iandunn]

Once Gutenberg supports FSE child themes, we should convert this into one

See #1, #2

• do Remove Sass wporg-parent-2021#1 before this, see Gutenberg: Convert to child theme #13 (comment)
• probably delete everything in https://github.com/WordPress/wporg-parent-2021, and replace it w/ the bits of the news theme that will be shared
• save_dependent_global_styles() should be updated to use the parent theme stylesheet rather than News, so that News-specific styles aren't loaded on other sites in the network
• lots of other tasks need to be identified

[iandunn]

It looks like some good progress is being made (see WordPress/gutenberg#34354), so we may be able to do this sooner than expected

[iandunn]

It'd be nice to do WordPress/wporg-parent-2021#1 before this. Once we have multiple themes, migrating becomes a much larger chore.

That's not a blocker though.