feat(conform-react): make error array only

[edmundhung]

Fix #212.

This is a follow-up of #228 and it expect error always be an array. The impact is mainly internal, except if you are using the base parse helper with custom resolve option, e.g:

function parseFormData(formData: FormData) {
  return parse<SignupForm>(formData, {
    resolve({ email, password, confirmPassword }) {
      const error: Record<string, string[]> = {};

      if (!email) {
        error.email = ['Email is required'];
      } else if (!email.includes('@')) {
        error.email = ['Email is invalid'];
      }

      if (!password) {
        error.password = ['Password is required'];
      }

      if (!confirmPassword) {
        error.confirmPassword = ['Confirm password is required'];
      } else if (confirmPassword !== password) {
        error.confirmPassword = ['Password does not match'];
      }

      if (error.email || error.password || error.confirmPassword) {
        return { error };
      }

      // Return the value only if no error
      return {
        value: {
          email,
          password,
          confirmPassword,
        },
      };
    },
  });
}