Async validation

[sholladay]

This is a feature request to allow the validate() function to return a Promise for the validation result. A common use case is to perform an API request to check whether a username is unique in the database.

export default function SignUpForm() {
    const [state, { text }] = useFormState();
    return (
        <input
            {...text({
                name: 'username',
                validate: async (value) => {
                    const result = await checkUniqueUsername(value);
                    return result;
                },
                validateOnBlur: true,
            })}
            required
        />
    );
};

[jfennell-techempower]

I would love this functionality for this exact use case,

[jfennell-techempower]

Similarly, allowing the other input event handler functions to also be async would be very helpful.