Custom validation checks do not support async implementation

[amiigas]

Async implementation of custom validation checks is not supported even though snippets mention this in @json-render/react documentation

https://json-render.dev/docs/validation#usage-with-react

    isUniqueEmail: async (value) => {
      const response = await fetch(`/api/check-email?email=${value}`);
      const { available } = await response.json();
      return available;
    },

https://json-render.dev/docs/api/react#validationprovider

type ValidationFunction = (value: unknown, args?: object) => boolean | Promise<boolean>;