[PoC] Add presentPaywall Expo Go support

[tonidero]

This is a working PoC of enabling usage of the presentPaywall function in Expo Go. There are some questionable decisions taken to make it work:

• The client needs to wrap their app in a WebViewPaywallProvider object so we can inject the modally presented paywall on demand. This is a relatively common pattern in RN, but it's not great we need to do this only for Expo Go :(. Other alternatives considered were adding some native functions that can be used to present the paywall on a window/VC on top of the RN UI which is also pretty complex and might require also a lot of wiring.
• Adds another optional peer dependency react-native-webview which needs to be included to make it work
• The WebView loads a different instance of purchases-js into the webview from unpkg. This can be dangerous since we basically have 2 instances of purchase-js running, which might not be in sync...

Screen_recording_20251127_151813.mp4