Update FieldProps to mimic formik handlers

[lukaskral]

This change fixes the type signature for the onChange handler, which can be called with a string as a 1st argument.

Example code:

import * as React from 'react';
import {Field, FieldProps} from 'formik';

import {TextAreaField} from 'my-own-ui';

interface IProps {
    id: string;
    label: string;
}

function TextArea({id, label}: IProps) {
    return (
        <Field
            name={id}
            render={({field}: FieldProps) => {
                return (
                      <TextAreaField
                          label={component.label || ''}
                          value={field.value}
                          onChange={(val: string) => field.onChange(val)}
                      />
                );
            }}
        />
    );
}

Up until now this wasn't possible in Typescript because field.onChange only accepts React.ChangeEvent<any>

[jaredpalmer]

LGTM.