theme.json schema: `ref` is not supported

[mikachan]

Description

In #41696, we added the ability to reference values from other locations in the tree. However, currently, this isn't valid theme.json markup because the reference is an object rather than a string.

Step-by-step reproduction instructions

Add the following markup to a theme.json file:

		"elements": {
			"button": {
				"color": {
					"background": {
						"ref": "styles.color.text"
					},
					"text": {
						"ref": "styles.color.background"
					}
				}
			}
		}

You should see a warning in your text editor that the ref is an incorrect type (Incorrect type. Expected "string".).

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes