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

[turbocrime]

Description

1. create any valid syncpack config json
2. add "$schema": "https://unpkg.com/syncpack@12.3.0/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 👍