Lack of support for React-Native-Web v0.12+ (usage of deprecated/removed ViewPropTypes)

### Version
- react-native-router-flux v4.2.0
- react v16.13.1
- react-native v0.63.2
- react-native-web v0.13.13
- expo v39.0.3

### Expected behaviour
Application using `react-native-router-flux` should render on web correctly (using `react-native-web` v0.12+), without any errors.

### Actual behaviour
`react-native-web` had removed support for `ViewPropTypes` since v0.12 (because Facebook has declared the `PropTypes` API as deprecated and suggests Flow/Typescript instead).

Because of that, any react-native library which use `ViewPropTypes` simply throws runtime error/s when running on web using `react-native-web` v0.12+.

In `react-native-router-flux`'s case, the error is thrown in `Router.js` at the specified line:
https://github.com/aksonov/react-native-router-flux/blob/b9377e3b39752cf19fa9f7d2c530543efd0be3db/src/Router.js#L135

Because `ViewPropTypes` doesn't exist on `react-native-web` v0.12+, the following error is thrown:
```
Router.js?d51e:124 Uncaught TypeError: Cannot read property 'style' of undefined
at eval (Router.js?d51e:124)
at Object../node_modules/react-native-router-flux/src/Router.js (renderer.js:7834)
at __webpack_require__ (renderer.js:791)
at fn (renderer.js:102)
at eval (index.js?1486:2)
at Object../node_modules/react-native-router-flux/src/index.js (renderer.js:7922)
at __webpack_require__ (renderer.js:791)
at fn (renderer.js:102)
at eval (content.tsx?d0ff:4)
at Object../src/screens/main/content.tsx (renderer.js:12130)
```

### Steps to reproduce
1. Create React-Native application using `expo-cli`
2. Ensure `react-native-web` is updated to latest version (or at least v0.12)
3. Add react-native-router-flux usage to the app
4. Run the app in web, using `yarn web` / `expo start --web`

### Possible solution / Quick fix
The above error can be solve by removing any `ViewPropTypes` usage from the library, or by mocking `ViewPropTypes` functionality whenever `ViewPropTypes` is `undefined`.

**For example, the following fix has been solved the above error in `Router.js` (line 135) for my case**:
`sceneStyle: ViewPropTypes ? ViewPropTypes.style : PropTypes.object`

This fix at `Router.js` seems to make `react-native-router-flux` function well with `react-native-web` v0.12+, but I don't sure if there are any more `ViewPropTypes` usage in the library.

However, mocking any `ViewPropTypes` usage with relevant `PropTypes` type will probably solve any issue like this