Font Library: simplify font collection schema

[matiasbenedetto]

What?

Simplify the font-collection.json schema.

Why?

After removing the not needed preview key from the theme.json schema, we added some complexity to the font-collection.json schema that was revealed as unnecessary and promoting more complex code down the line, so in this PR, we are reverting part of the complexity added.

How?

Simplify the font-collection.json schema to make it look like theme.json font families but with the preview key.

Testing Instructions

Create a JSON file in vscode or other editor with JSON Schema validation and autocomplete.
Add this content to the file:

{
    "$schema": "https://raw.githubusercontent.com/WordPress/gutenberg/ed06197dc046f77a38d290182c632c7d20af7d71/schemas/json/font-collection.json",
    "name": "My Font Collection",
    "slug": "my-font-collection",
    "description": "A collection of my favorite fonts",
    "font_families": [
        {
            "font_family_settings": {
                "fontFamily": "Roboto",
                "name": "Roboto",
                "slug": "roboto",
                "fontFace": [
                    {
                        "fontFamily": "Roboto",
                        "fontStyle": "normal",
                        "fontWeight": "400",
                        "src": "https://fonts.gstatic.com/s/roboto/v20/KFOmCnqEu92Fr1Mu4mxP.ttf",
                        "preview": "https://fonts.gstatic.com/s/roboto/v20/KFOmCnqEu92Fr1Mu4mxP.ttf"
                    }
                ]
            },
            "categories": [
                "sans-serif"
            ]
        }
    ],
    "categories": [
        {
            "name": "Sans serif",
            "slug": "sans-serif"
        }
    ]
}

You should not see any validation error.
If you add keys not present in the schema you should see validation errors.

[creativecoder]

See comments for a couple of nitpicks.

It looks like the font_family_settings description also needs to be updated. It says "Font family settings similar to theme.json but without fontFace key.", and that's no longer correct.

Otherwise looks good

• ✅ schema is valid
• ✅ Google fonts collection is valid against the schema
• ✅ properties from theme.json match here with correct types and descriptions

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Core SVN

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Props: mmaattiiaass, grantmkin.

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Co-authored-by: matiasbenedetto <mmaattiiaass@git.wordpress.org>
Co-authored-by: creativecoder <grantmkin@git.wordpress.org>

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[creativecoder]

I pushed a couple of small updates, the main one being the font_family_settings description. This looks good to go!