fix(jsonschema): add $schema property

[mst-mkt]

⚡ Summary

In the current jsonschema, specifying $schema results in a warning as shown in the following image.

This is because "additionalProperties": false and there is no $schema in properties.

To resolve this, the $schema property was added.
This approach has been observed in other projects as well, such as @biomejs/biome.

    "properties": {
+     "$schema": {
+       "type": "string"
+     },
    },

I am not very familiar with Go, so I would appreciate any corrections or suggestions.

☑️ Checklist

• Check locally
• Add tests
• Add documentation