Theme JSON schema: Add defaultPresets property to shadow

[t-hamano]

Related to: #46813

What?

This PR adds the defaultPresets property to the shadow property of the theme.json schema.

Why?

The dev note on the shadow property does not mention defaultPresets. However, I would expect this property to be controllable via theme.json.

How?

I have set additionalProperties to false in addition to adding the shadow property.

Testing Instructions

Create a JSON file that references this PR with the $schema property:

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

• Under the shadow property, confirm the defaultPresets property is selectable.
• Confirm that the code editor shows an error if you define properties other than defaultPresets and presets.

[t-hamano]

I have found that a similar schema fix was backported to wp/6.2 branch in #48948. I don't know if it is possible to backport it now, but I will give it a backport label just in case.

[github-actions]

Flaky tests detected in 793daa7.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4468654202
📝 Reported issues:

• [Flaky Test] can create template parts via the block placeholder start blank option #49016 in /test/e2e/specs/site-editor/template-part.spec.js
• [Flaky Test] should be able to create an hierarchical post without title support #36939 in specs/editor/plugins/custom-post-types.test.js

[ajlende]

This looks good to me. I found where shadow.defaultPresets is used.

And thanks for tightening up the additionalProperties for shadow. That's a good addition.

[Mamaduka]

I just cherry-picked this PR to the wp/6.2 branch to get it included in the next release: dcfb709