Adding customCancelButton and customConfirmButton types

[luizaugustoventura]

Thank you guys for an amazing lib!!

PS: I closed this PR because I had the wrong email in the commit, so the commit's signature was unverified. Consider this Pull Request instead. I am sorry for that!

Overview

The library is missing the CancelButton and ConfirmButton components for the iOS only properties customCancelButtonIOS and customConfirmButtonIOS. I also found a good idea to export their styles, just in case someone (myself included) wants to do some slight changes.

All I mentioned works fine, but whenever you try to import any of these two components or their styles you get some red on your code editor. That's because the declaration file is missing these elements. So all I did was I exported from ìndex.d.ts the CancelButton and ConfirmButton components, told the declaration file the customConfirmButtonIOS and customCancelButtonIOS properties were a React.FunctionComponenet<props> with some extra props. I also exported the style types of both components.

Test Plan

There is not much to do actually, all I did was passing the buttonTextColorIOS property to the custom button I was using. For example:

import ModalDateTimePicker, { CancelButton };

<ModalDateTimePicker
    // ... <normally used props>
    customCancelButtonIOS={(props) => <CancelButton {...props} buttonTextColorIOS="red"  />}
/>

If you want to change the button styles, you might want to do something like that:

import ModalDateTimePicker, { CancelButton, CancelButtonStylePropTypes, cancelButtonStyles };

const customStyles: CancelButtonStylePropTypes = {
    ...cancelButtonStyles,
    text: {
        ...cancelButtonStyles.text,
        color: 'red',
    },
};

<ModalDateTimePicker
    // ... <normally used props>
    customCancelButtonIOS={(props) => <CancelButton {...props} style={customStyles} />}
/>