Visual Studio Code schema support

With generateSchema: true (default), the preprocessor writes .svelte-kit/markdoc_schema.js.

You can point the official Markdoc VS Code extension to that file in markdoc.config.json:

[
    {
        "id": "my-site",
        "path": "src/routes",
        "schema": {
            "path": ".svelte-kit/markdoc_schema.js",
            "type": "esm",
            "property": "default",
            "watch": true
        }
    }
]