Rename experimental-theme.json to theme.json and add a fallback for backwards-compatibility

[aristath]

Description

With the FSE MVP closing in, we should rename experimental-theme.json to theme.json.
This PR makes the necessary changes to documentation files, and also in the JSON resolver.
For backwards-compatibility and to avoid existing FSE themes from breaking, this PR also adds a fallback so if theme.json can't be found it tries to use experimental-theme.json.

How has this been tested?

Tested with an existing FSE theme and nothing seems to be breaking. Then renamed the experimental-theme.json file to theme.json inside the theme root folder, and again nothing breaks.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• [-] I've tested my changes with keyboard and screen readers.
• [-] My code has proper inline documentation.
• [-] I've included developer documentation if appropriate.
• [-] I've updated all React Native files affected by any refactorings/renamings in this PR.

[aristath]

cc @JustinyAhin for a review of the docs changes

[youknowriad]

Thanks for the PR Ari, It's great to have this ready. I think we should hold a bit on this PR until we make the necessary changes suggested in #29891

[aristath]

Thanks for the PR Ari, It's great to have this ready. I think we should hold a bit on this PR until we make the necessary changes suggested in #29891

Sure, though with the fallback in https://github.com/WordPress/gutenberg/pull/29981/files#diff-41112e4d1e1b9d7fe224c7c598fb56d7ed165d57335008129e1c4658252fb037 I don't think anything will break 😉

[github-actions]

Size Change: 0 B

Total Size: 1.31 MB

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[youknowriad]

It's more about the "format" for me and also a "communication" aspect. If we decide that we need some small breaking change in the format. For an experimental file, it's clearly mentioned on our docs that changes are expected https://developer.wordpress.org/block-editor/contributors/code/coding-guidelines/#experimental-and-unstable-apis

[aristath]

Makes sense 👍
Thank you for the clarifications!

[youknowriad]

Time to get this one refreshed and merge. The new format is in. Thanks for the patience :)

[youknowriad]

Thanks, the fallback is a good addition :)