Support $schema property in prismic.config.json

[angeloashmore]

Description

Add a $schema property to prismic.config.json so editors like VS Code can provide autocomplete and validation.

Approach

1. Generate a JSON Schema from the Zod ConfigSchema (see the docs) and ship it in the published package.
2. Accept $schema as an optional passthrough in the Zod schema so it doesn't fail validation.
3. Write $schema into the config during createConfig(), pointing to the schema file in node_modules (e.g., ./node_modules/@prismicio/cli/prismic.config.schema.json).