Support server-only validations

[brophdawg11]

Allow a server-only set of custom validations to be passed into validateServerFormData:

import type { ServerOnlyCustomValidations } from 'remix-validity-state'
import { validateServerFormData } from 'remix-validity-state'

let formDefinition: FormSchema = {
  inputs: {
    emailAddress: {
      validationAttrs: {
        type: "email",
        required: true,
      },
      customValidations: {
        // always "valid" in shared validations
        uniqueEmail: () => true,
      },
      errorMessages: {
        uniqueEmail: () => 'Email address already in use!'',
      },
    },
  }
};

const serverCustomValidations: ServerOnlyCustomValidations<FormSchema> = {
  emailAddress: {
    async uniqueEmail(value) {
      let isUnique = await checkEmail(value);
      return isUnique;
    },
  },
};

export async function action({ request }: ActionArgs) {
  const formData = await request.formData();
  const serverFormInfo = await validateServerFormData(
    formData,
    formDefinitions,
    serverCustomValidations
  );
  // serverFormInfo.valid here will be reflective of the custom server-only
  // validation and will leverage your shared `errorMessages`
  if (!serverFormInfo.valid) {
    return json({ serverFormInfo });
  }
  // ...
}

Closes #25