Update shape of theme.json: settings & styles as top-level keys

[oandregal]

📢 The changes have landed in Gutenberg master, so this is ready

The goal of this PR is to serve as a testbed for the upcoming changes at WordPress/gutenberg#28110

[oandregal]

WordPress/gutenberg#28110 is ready and will be merged soon (probably tomorrow). Should this PR be merged soon after so this repo is in sync with Gutenberg's main branch?

cc @kjellr @scruffian

[carolinan]

If we do that, the documentation must be updated to reflect that this version of the theme requires the development version of Gutenberg.

It needs to be made very clear to users, including those who are testing FSE within the FSE outreach program, which version of the theme to use for testing, together with which version of Gutenberg.

[oandregal]

Updated this to make it work with WordPress/gutenberg#28533

[oandregal]

I figured this PR could be useful for people to match the changes in WordPress/gutenberg#28110 given that it has landed in master so I scoped it down to it.

Not sure how's the process but this can land safely.

There's now a separate #182 to serve a testbed for a new Gutenberg PR.

[kjellr]

My suggestion is that we merge this in to the version here (alongside a version bump), but do not upload this version to the theme repository until it's been merged into the plugin.

I do think some sort of clarifying note in this repo may be helpful — something along the lines of "The version of the theme in this repository may rely on Gutenberg features that have not yet made it into an official release."

[oandregal]

I wanted to share that the second PR to update the theme.json shape also landed in Gutenberg and will be part of 9.9 WordPress/gutenberg#28533 This will still require the plugin to work and won't be part of WordPress 5.7.

Given that the two Gutenberg PRs already landed I think it's best to close this PR and land #182 instead (it has all the changes already).