Theme JSON schema: Fix "not allowed error" in settings property

[t-hamano]

Related to #54509

What?

This PR fixes an issue in the theme.json schema that causes errors where all properties in the settings property are not allowed except ligntbox.

Before	After

Why?

#54509 added a reference to the Lightbox schema (settingsPropertiesLightbox) to the settings property. settingsPropertiesLightbox defines additionalProperties :true at the root level of the object, so this applies to the settings property itself. As a result, properties other than the lightbox property are no longer allowed.

How?

Moved additionalProperties inside the lightbox property. This will solve the problem with the settings property, and will also output errors for properties that are not allowed in the lightbox property. My guess is that this is expected behavior.

Testing Instructions

Create a json file like the one below locally:

{
	"$schema": "https://raw.githubusercontent.com/WordPress/gutenberg/schema/fix-settings-definition/schemas/json/theme.json",
	"version": 2,
	"settings": {
		"lightbox": {
		}
	}
}

• Confirm that no error occurs when you enter properties other than the lightbox property directly under settings.
• Confirm that an error occurs when you enter properties other than allowEditing and enabled in the settings.lightbox property.

[github-actions]

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[aristath]

LGTM and fixes the issue 👍

[t-hamano]

@aristath Thanks for the review 🚀

[walbo]

LGTM 👍