Ensure the form is validated while creating a kernelspec

[echarles]

For now, we hide the standard react-json-form button to only display the JupyterLab buttons

https://github.com/Quansight/ksmm/blob/9d92b59f12f432dd5c78c6b9d5e511e128b01f0a/src/widget.tsx#L96-L107

The standard react-json-form button ensures validation, but the JupyterLab buttons does not.

There should be a way to call the validation and not accept the creation if the form is not valid. The validation ways are documented on https://react-jsonschema-form.readthedocs.io/en/latest/usage/validation/

I have quickly tried liveValidate attribute but it does not seem to work.