Add core/icon block to theme.json schema

[t-hamano]

What?

This PR adds a missing Icon block entry to the JSON theme schema.

Why?

To improve developer experience.

Testing Instructions

Use test/emptytheme/theme.json to confirm that core/icon appears in the following three places:

• settings.blocks
• styles.blocks
• styles.variations.{variation-name}.blocks

{
	"$schema": "../../schemas/json/theme.json",
	"version": 3,
	"settings": {
		"blocks": {
			"core/icon"
		}
	},
	"styles": {
		"blocks": {
			"core/"
		},
		"variations": {
			"my-variation": {
				"blocks": {
					"core/"
				}
			}
		}
	}
}

Screenshots or screencast

[github-actions]

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

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[t-hamano]

Since the need for this change is clear and it is just a schema change, I'd like to merge this PR.

[github-actions]

I just cherry-picked this PR to the wp/7.0 branch to get it included in the next release: d69a544