### New features
* 📱 Initial support for cross-platform RevenueCat Paywalls 🐾 🧱 (#852)

#### Instructions:
- For Android, you need to change your `MainActivity` to subclass
`FlutterFragmentActivity` instead of `FlutterActivity`. Also, the min
sdk version of the new package is `24`. Please make sure your app's
`android/build.gradle` minSdkVersion has that or a higher version.
- Add `purchases-ui-flutter` in your `pubspec.yaml`:
```yaml
dependencies:
  purchases_ui_flutter: 6.15.0-beta.4
```

#### Usage:
```dart
import 'package:purchases_ui_flutter/purchases_ui_flutter.dart';

await RevenueCatUI.presentPaywallIfNeeded("pro");
```

#### Limitations:
- Currently only full screen paywalls are supported
- There is no way to detect paywall events other than using
`addCustomerInfoUpdateListener`

#### Breaking changes from previous beta:
- Paywalls functionality has been extracted into a new dependency called
purchases_ui_flutter. Add purchases_ui_flutter to your pubspec.yaml.
Before this release, it was part of the main RevenueCat SDK
`purchases_flutter`.
- A new import is required to use this functionality: `import
'package:purchases_ui_flutter/purchases_ui_flutter.dart';`
- Usage has changed the class from `Purchases.presentPaywall` to
`RevenueCatUI.presentPaywall`.
- `presentPaywall` and `presentPaywallIfNeeded` now don't return a
result. To detect purchases, please use `Purchases.getCustomerInfo` from
the `purchases_flutter` SDK. A return value will be added in future
releases.

### Other Changes
* Fix pub score for `purchases-ui-flutter` (#924) via Toni Rico
(@tonidero)
* Update `paywalls` to latest `main` (#923) via Toni Rico (@tonidero)
* Rename `purchases_flutter_ui` to `purchases_ui_flutter` (#921) via
Toni Rico (@tonidero)
* Separate paywalls into a different package (#919) via Toni Rico
(@tonidero)
* Remove result from presentPaywall methods (#916) via Toni Rico
(@tonidero)
* Update to use PHC PaywallHelpers instead of custom activity (#903) via
Toni Rico (@tonidero)