Add format attribute to the palette's color property

[unscripted]

What?

Enhances the JSON schema for WordPress theme.json to enable color swatches and the color picker functionality for hex values within JSON in VS Code.

This PR adds the format attribute to the palette's color property and sets color as the value.

Why?

This change improves the developer experience by visually displaying color values through swatches and enabling a color picker in VS Code. It simplifies working with color-related properties in theme.json and style variations, reducing errors and enhancing productivity.

How?

Updates the JSON schema to include the "format": "color" attribute to the palette's color property.

Testing Instructions

1. Open a WordPress project in VS Code.
2. Add or update a theme.json file with color-related settings (e.g., settings.color.palette).
3. Verify that color swatches are displayed next to color values in VS Code.
4. Ensure the color picker activates when editing color values.
5. Confirm compatibility with various color formats, including:
   • Hex (#3858e9)
   • Hex with alpha (#3858e950)

Test theme.json

{
	"settings": {
		"color": {
			"palette": [
				{
					"color": "#3858e9",
					"name": "Hex",
					"slug": "hex"
				},
				{
					"color": "#3858e950",
					"name": "Hex with Alpha",
					"slug": "hex-alpha"
				}
			]
		},
	}
}

Screenshots or screencast

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: unscripted <[email protected]>
Co-authored-by: carolinan <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[carolinan]

Note to testers: you need to point the $schema in theme.json to the updated file.
"$schema": "../../plugins/gutenberg/schemas/json/theme.json", or similar.

I liked the experience of selecting the colors in theme.json in VSCode, but I wont pretend to understand how it works 😄 so this is not a code review.