block.json schema: Add role field

[t-hamano]

See: #65484 (comment)
Dev note: https://make.wordpress.org/core/2024/10/20/miscellaneous-block-editor-changes-in-wordpress-6-7/#stabilized-role-property-for-block-attributes

What?

Add a missing role field.

Why?

The role field was stabilized in WordPress 6.7, but was not exposed in the schema.

Testing Instructions

Create the following JSON file to test code editor assistance:

{
	"$schema": "https://raw.githubusercontent.com/WordPress/gutenberg/refs/heads/block-json-schema-role/schemas/json/block.json",
	"apiVersion": 3,
	"title": "Test",
	"name": "core/test",
	"attributes": {
		"test": {
			"type": "string"
		}
	}
}

Screenshots or screencast

[github-actions]

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[Mamaduka]

Thanks, @t-hamano!