Add NavigationCard constants

[dingbat]

Currently any use with react-router-native fails because it expects several classes/functions to be defined on NavigationExperimental.Card (see in react-native here).

It also expects forHorizontal and forVertical in CardStackStyleInterpolator and forHorizontal in PagerStyleInterpolator, all on load.

This PR adds all those mocks to the existing NavigationExperimental mock.

Thanks for the hard work put into this library!

[dingbat]

@RealOrangeOne Lint wanted it written as a stateless function. Let me know what you think.

[RealOrangeOne]

@dingbat I understand why. Instead of adding the keys after the fact, could you use the static block in a component to define them, would be much easier, and it shouldnt complain then

[dingbat]

@RealOrangeOne I'm not sure what you meant by using a static block, but I added the constants as static fields on the component class. Thanks for the good feedback.

[RealOrangeOne]

Almost done. Can you remove the brackets around the fat-arrow functions, keep things a little cleaner, as they aren't needed. Then we can ship! :shipit:

[dingbat]

Oh, the parentheses? They are needed because the functions return an empty object, so it's ambiguous with the omitted-bracket function syntax.

Actually now that I think about it, the library doesn't call the functions, it just references/asserts that they're present, so we could even just do:

static CardStackPanResponder = CardStackPanResponder;
static CardStackStyleInterpolator = {
  forHorizontal: {},
  forVertical: {},
};
static PagerPanResponder = PagerPanResponder;
static PagerStyleInterpolator = {
  forHorizontal: {},
};

[RealOrangeOne]

ah, didnt realise they were returning empty objects. thought they werent returning. In that case! :shipit: