Introduce onCanGoBackChange & onCanGoForwardChange

[mkhazov]

WKWebview does not trigger onProgress and onNavigationStateChange when navigation occurs in SPA (e.g. React app with react-router navigation).

The issue is solved by adding key-value observers for canGoBack and canGoForward properties and passing them to corresponding functions passed as props from React Native.

Usage:

<WKWebview
  onCanGoBackChange={ (canGoBack: boolean) => {} }
  onCanGoForwardChange={ (canGoForward: boolean) => {} }
/>