fix(config): add `$schema` property as optional string

[turbocrime]

Description

1. create any valid syncpack config json
2. add "$schema": "https://unpkg.com/[email protected]/dist/schema.json" to your config
3. the schema fails to validate due to the presence of a $schema field

Suggested Solution

include "$schema": { "type": "string" } in your schema properties

[JamieMason]

Thanks @turbocrime 👍

[JamieMason]

Released in 12.4.0