feat: support header title color

[agarant]

Currently it was not possible to set a title color different than the tint color. This PR keeps the tintColor as the default header title color but also uses the headerStyle.color if defined.

[Collin3]

I think you should be able to achieve this by setting your screen options to something like the object below. This is how I was able to style the back button, but set the title text to a different color. I'm using the latest version (2.5.0).

{
  headerTintColor: yourTintColorHere,
  headerTitleStyle: {
    color: yourTitleColorHere
  }
}

[agarant]

@Collin3 Thanks for suggesting a workaround.

However, I still think that the fix is relevant as it is makes the api more intuitive and consistent. @WoLewicki do you mind taking a quick look at it. It is a 1 line PR. :)

[WoLewicki]

Thanks for your contribution! Looks good, I am merging it.

[hirbod]

Maybe a hint in the native-stack readme?

[WoLewicki]

Could someone make a PR with an update to the README and screens.d.ts that it would properly show the available options based on createNativeStackNavigator.js? I would love to merge a PR with it.

[agarant]

@WoLewicki of course, I can make the update in #454, updating the required docs and types for the largetTitle property at the same time.