Fix typescript error due to schema type

[klaseca]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

Description:

Schema type was changed in pull request #134. One of the available schema types was ObjectSchema from fluent-json-schema package. Since fluent-json-schema is a development dependency, we cannot use it for production code

Now, if we do not install fluent-json-schema manually, we will get the wrong type for schema, or an error if we use "skipLibCheck": false in the typescript config file

After this fix, we can still use fluent-json-schema, but we will not get wrong type or error if this package is missing

[mcollina]

lgtm